ABSTRACT
A number of plants grown in the full daylight of a greenhouse have been exposed daily to the radiations of an Ulviarc mercury vapour lamp for very short time intervals, varying from 10 minutes to 30 seconds in the different experiments.
Stunting and other formal and formative effects were seen with intervals of 1 minute and upwards.
Exposures of 30 seconds on seedlings of Trifolium six weeks old appear to have no stunting effect and a favourable after-effect.
Exposures of 2 minutes given to plants of Voandzeia receiving natural illumination for 7 hours or 5 hours daily have a more serious effect than when given to a plant receiving a 12-hour day.
INTRODUCTION
Of recent years great interest has been aroused by the repeated demonstration of the biological effects of ultra-violet light especially in connection with its effect on animal tissues. The range of wave-length concerned is mainly from 3100 to 2500 Ångstrom units; rays of shorter wave-length (λ = 2900 to 2500) are bactericidal and almost non-penetrating to tissues; those of longer wave-length have a greater power of penetration and produce under appropriate conditions a general tonic effect accompanied probably in most cases by stimulation of cell growth and fundamental metabolic processes (Hill and Eidenow 1923, Harris 1925). Photo-chemical effects of ultra-violet radiations are also becoming widely known, but there is little available evidence as to their effects in connection with plant life.
The ultra-violet rays of ordinary sunlight vary from 3000-4000 Ångstrom units, but are absorbed by the atmosphere when it is humid or smoke-laden so that none may reach the surface of the earth. In places where the air is clear and the sun brilliant, the animal and plant life must be frequently exposed to ultra-violet as well as visible light. It is possible that this difference in natural illumination may bear on the survival of plants brought from sunny regions to climates such as our own, especially under greenhouse conditions. In the latter case much of the radiation will be absorbed, the proportion absorbed being greater as the wave-length increases; according to Coblentz (1926) about 60 per cent, of radiations of λ 3400 will be transmitted, whilst about 85 per cent, of λ 3600 and upwards will be transmitted through ordinary greenhouse glass. The initiation of the following experiments was with the object of finding some way, if possible, to facilitate the cultivation of experimental plants of Arachis in the greenhouse in England for purposes of research work in other directions. The problem is however more complex than would appear at first sight, involving the question of duration of the daily natural illumination (Garner and Allard 1920, Tincker 1925), as well as the influence of light of different wave-length and intensity.
The object of the present investigation was to observe the influence of exposing plants grown otherwise under normal conditions to the full light of a quartz mercury vapour lamp for short intervals, and at varying distance from the source of light. The experiments were carried out mostly in August, September and October, and in the two latter months there were many foggy dull days when no ultra-violet light reached the plants from solar radiation.
CONSIDERATION OF RESULTS OF PREVIOUS WORKERS
Various attempts have been made to investigate the influence Of radiation of a limited region of the spectrum on plants. Schanz (1919) and others grew plants under natural light from which some of the coloured components were removed by means of suitable screens. In his experiments with beans, cucumbers and other plants the best results culturally were obtained when the plants were screened so that the blue, violet and ultra-violet end of the spectrum was removed. It was concluded that these radiations are not beneficial and it was suggested that they should be eliminated by the use of “Euphos” glass in order to get the best results in greenhouse cultivation.
In 1925 a similar series of experiments was reported by Popp, of which the fuller account appeared in 1926. In these experiments pot plants were grown in light from limited regions of the solar spectrum, isolated by means of appropriate screens and adjusted to be of about the same effective intensity. It was found that whereas taller plants were formed under glass from which light of short wave-length was excluded, yet the fresh and dry weight was always much less than that in plants grown in full insolation: the beneficial cultural effects noted by Schanz may thus have only been superficial. The blue violet end of the spectrum was regarded as necessary for normal growth, antagonising the otherwise harmful effects of illumination limited to the red and orange rays: the ultra-violet being non-essential.
Popp has also, in common with previous workers, studied the influence of full illumination with an artificial source of ultra-violet light. The details of these experiments (1925) are not yet available, but he used both a quartz mercury vapour lamp and a carbon arc and observed their effects on germination and subsequent growth of seedlings. Exposures of more than 2 hours with the carbon arc lamp inhibited growth and development, finally causing death. During germination the cotyledons escaped with difficulty from the seed coats, but once expanded seemed uninjured. Young leaves of mustard, sunflower and other seedlings were killed in 1–2 days. No information is as yet available as to the distance between the plants and the light source or other experimental conditions.
A number of experiments were also made by Russell and Russell (1925) with seedlings of mustard exposed to light from a Hewittic mercury vapour lamp. The exposures varied in different cases from 5 to 60 minutes daily and were given at a distance of 2 feet. In every case there was marked stunting of growth and foliage and this was greater in the case of seedlings grown otherwise in darkness and less when given to plants accustomed to normal periods of daylight. The experiments were not carried far enough to give indications as to the resultant effects on structure and reproduction. The Hewittic mercury vapour lamp is very similar to the Ulviarc lamp which was used in the present investigation. It is of course very different from the carbon arc lamp used by Popp for the majority of his experiments, both in nature and in intensity of radiation. In our experience, the Ulviarc lamp gives an appreciable local heating up effect for intervals longer than 2 or 3 minutes and at a distance of 2 feet: the authors seem to have taken no precautions in this respect.
The results of previous workers cannot be closely compared owing to the different experimental conditions and means of illumination employed. In general it may be said that the influence of ultra-violet rays on plants has been found unfavourable, and that this effect is more pronounced in plants grown otherwise in darkness than it is in plants exposed to normal daylight. It remains to be seen whether this is always the case, or whether, as in animals, sufficiently short exposures to an artificial source of ultra-violet light may prove to be stimulating or in any way beneficial. In the latter case, the stunting effects on plant growth so often noted by previous workers might perhaps be compared with the violent reaction occurring in animals or in human beings after too large a dose of ultra-violet radiation.
SOURCE OF LIGHT IN THE PRESENT INVESTIGATION
The illumination was given in short daily doses, varying from to 15 minutes in the different experiments, by means of an Ulviarc quartz mercury vapour lamp obtained from the Hewittic Electric Co. (consumption 3·5 amps resistance of 150 ohms across arc). The expenses incurred in this connection have been met by a grant from the Research Committee of the British Association.
When the experiments were ended, one of the twin lamps used was sent to the National Physical Laboratory and the nature of its radiation was tested: no measurable difference was found between the radiations at a distance of i metre and 4 metres from the lamp. The spectrum was photographed (cp. Pl. II) and found to give lines representing λ 2260 and upwards including some in the visible region. The radiations were variable in nature and intensity for the first few minutes after lighting up the lamp but after that reached a steady and practically constant value. A typical behaviour is shown in Fig. 1 where light consisting mainly of two wave-lengths was tested—λ 5461 in the green, and λ 3650 in the near ultra-violet. This had already been suspected, and in all but the earliest experiments an interval of 10 minutes after lighting up was observed before using the lamp. It will be seen from Fig. 1 that a simple approximate numerical correction can be applied to exposures which were given with the lamp newly lit, the effective illumination being reckoned from the graph of λ 3650.
EXPERIMENTAL PRECAUTIONS
Conditions of temperature and moisture were the same for control and experimental plants and were observed daily, but no special precautions were taken to keep them constant. The possible drying of the air and heating effect of the lamp were avoided as far as possible by shortening the times of exposure or dividing them so that three or four exposures a day were needed for a total exposure of 5 minutes or more. The plants were grown in pots in ordinary daylight and when illuminated were grouped directly under the source of light, at a distance of 2 or 3 feet in most cases, or 8 feet on occasion. Inequality of illumination owing to position within the circle of light was compensated by appropriate turning or shifting of the pots.
GENERAL RESULTS
Investigations of this kind carried out by Miss K. Ritson at Kew on Arachis, Voandzeia, Pelargonium, Trifolium and other plants and parallel ones at Bedford College, Regent’s Park, carried out independently by Miss A. Westbrook on Arachis and Voandzeia, gave results which were on the whole very similar: involving in addition to a general stunting in growth, a number of other changes in form and structure. For convenience, each set of experiments will be described separately.
INVESTIGATIONS CARRIED OUT BY K. RITSON, B.Sc., AT KEW GARDENS
August-November, 1926.
A. Experiments with Arachis and Voandzeia
Seeds of Arachis hypogaea L. and Voandzeia subterranea Thou, (both geocarpic leguminous plants) were generously supplied by the Director of Agriculture for the Gambia, through the kind offices of Dr Chipp, Assistant Director of the Royal Gardens, Kew. Seedlings were raised in heat two together in 5-inch pots and transferred to one of the cool greenhouses at the Royal Gardens, Kew, by the kind permission of the Director. The temperature here usually averaged 60° F. on cloudy and 75° F. on sunny days: in colder weather the house was slightly heated, giving a temperature of 55–58° F. The benches used were covered with a layer of small coke kept moist by watering daily. Some plants were started in March 1926; others early in July, and both lots flowered in August and began to set seed. Some of the plants sown in March were given illumination daily1 for 15 minutes from the time of lighting with the Ulviarc lamp, equivalent to about 13 minutes’ full illumination and at a distance of 2 feet from the plants. After two days this was given at three intervals and then equalled about 7·5 minutes’ full consecutive illumination. The leaves soon became curled and yellowish; many fell and by the sixth day the plants appeared almost dead. It was noticed that at 2 feet distance there was a perceptible heat effect, which might partly account for these results and all subsequent experiments were made at a distance of 3 feet (or more) when no temperature change was registered at the level of the plant by an ordinary mercury thermometer during the short times of exposure. Parallel experiments with seedlings of Voandzeia gave very similar results, under the same conditions and in the same time, in August.
B. Experiments with Pelargonium, Coleus, Fuchsia, etc
A fresh attempt was then made using cuttings of Coleus and pot plants of Pelargonium, and other genera, all being either in flower or about to produce” inflorescences in the normal course. These plants were divided into groups, representing one control and two or more experimental plants. In these plants the attempt was made to accustom them gradually to ultra-violet light: the illumination was the same for the different plants and may be summarised as follows:
These short exposures given at a distance of 3 feet from the lamp and only for 5 days out of every 7 were sufficient to cause a number of well-marked changes in the plants used—(1) Pelargonium (hybrid)2 A., (2) Coleus Blumei Benth., (3) Fuchsia splendens Zuch., (4) Abutilon Savitzii, (5) Salvia splendens Ker-Gawl. In the first period (September 1–14) no change could be seen in any but the Pelargoniums in which the experimental plants seemed to be flowering more freely than the controls. At an early date in the second period nearly all the leaves of the irradiated Pelargonium, Coleus, Fuchsia and Abutilon had fallen, while the control plants had lost but few. Early in October the Pelargoniums and Salvias had lost all their leaves, the former slowly developing new ones.
One of the experimental plants of Pelargonium, bare of leaves by October 4, was given no further ultra-violet light. The other, bare by October 11, was still given exposures to the Ulviarc lamp twice daily (Table I): both put forth new leaves, those of the still illuminated plants being darker green in colour and more crumpled in appearance than the others. On October 28, all the leaves were removed from a branch of each plant and the regeneration of the foliage again observed. The illuminated plant made leaves which were at first more backward than those of the non-illuminated plant. From Nov. 1 the exposure was reduced to 30 seconds of full illumination daily and the young leaves of the irradiated plant developed quickly, the largest now surpassing those of the non-illuminated plant.
Two boxes of Coleus Blumei were also used, each containing 10 well-rooted cuttings, one being irradiated daily as in Table I and one serving as a control. In the first period no diiference was to be observed, but with increased illumination for 10 days from September 14, already the growth of the experimental plants was checked and the leaves, originally bright green with a red patch in the centre, had lost their anthocyanin and had become a dull dark green with a little yellowish colour near the base of the lamina: the leaves of the control were still bright green with vivid red and yellow variegation. The average height was 16·7 cm. in the controls as against 11·1 cm. in the experimental plants, all originally having been about 8 cm. in height. Other changes noted in the irradiated plants were delayed flowering (Coleus) and withering of flower buds which had formed (Pelargonium A., Fuchsia), while the control plants were flowering freely.
These results are very striking when one considers the brief intervals of illumination allowed. Their interpretation is however complicated by the variable times of previous irradiation and also in the case of Pelargonium A., by the double loss of foliage which may have seriously impaired the vigour of the plants. Additional observations are needed before an interpretation can safely be advanced.
Further experiments were made on the influence of short periods of illumination on germination and subsequent growth of Trifolium subterraneum. The seeds were obtained by courtesy of Messrs Sutton, of Reading, and were sown in two pans of soil, watered and kept covered with glass until the cotyledons had appeared above ground. Both were given full daylight within the greenhouse and in addition the experimental pan was irradiated daily with the Ulviarc lamp at a distance of 3 feet. The times of illumination in the first lot (A) varied somewhat; in (B) the illumination throughout was 30 seconds at full intensity of radiation. These are summarised in tabular form:
The seeds were sown on September 21. In the first period of group (A) (Table II) the effective illumination was probably about 8–10 seconds daily, assuming that the lamp behaved in a regular way at each time of lighting up (p. 141). The result was seen in a backward germination—55 seedlings having appeared in the experimental and 120 in the control pan: the ungerminated seeds came up however a few days later. The seedlings were now transplanted (Oct. 5), 8 or 9 to a 5-inch pot and for the next 28 days the exposure was increased to 5 minutes in the illuminated plants: a second photograph taken at this stage (Oct. 14) shows the more backward condition of the irradiated plants, the cotyledons being either still enclosed within the testa or scarcely opened and the hypocotyls shorter than in the control plants: the experimental plants were also a darker green than the controls. On October 25 a third photograph was taken: at this stage 54 out of 75 control seedlings had a third leaf developed (i.e. 72 per cent.) whereas only 19 out of 79 illuminated seedlings had three leaves (i.e. 24 per cent.). On November 29 all the seedlings had stopped growing and both lots died, the experimental before the end of the month and the controls by December 20.
On November 1, when the seedlings (lot A) were 6 weeks old, ten of the controls were selected of the same size and appearance and were potted separately. Five of these remained as controls and five were used as experimental plants (lot B) and were given 30 seconds’ illumination daily of full intensity at a distance of 8 feet from the source of light. This was continued for 4 weeks and allowing for week-ends, included 21 days of illumination. Growth was at a standstill, as was not surprising for the time of year, but the illuminated plants were noticeably greener than the controls at the end. The illumination was now discontinued (Table II), but the improvement in the experimental plants continued and at the time of writing (February 1927) has still been maintained, whereas three of the control plants have died and the remaining two are feebler and smaller than the experimental plants.
Thus with seedlings of Trifolium subterraneum grown in winter an exposure of 10-12 seconds daily caused delayed germination and when followed by 5 minutes daily illumination resulted in marked stunting; but exposure of normal seedlings already 6 weeks old to daily full illumination for 30 seconds resulted in an after effect of increased vigour: it should be noted however that the illumination was given during a month of dull weather when little sunlight was reaching the plants.
Typical seedlings of Trifolium subterraneum (lot A) were selected from the pot at the stage photographed on October 25, and the external features and internal structure of the experimental and control compared. Externally the experimental plant showed shorter roots with fewer laterals; leaves with shorter petioles, the laminae thin, curled and more or less imperfectly expanded (Fig. 2 b). Internally, the experimental seedling showed considerable changes in the structure of the lamina in sections taken from comparable positions on corresponding leaves. The most striking changes were seen in the collapse—partial or complete—of the epidermal cells; lack of differentiation and greater compactness of the mesophyll (Fig. 2 e,f). As in the experiments of Ursprung and Blum (1917) the guard cells were always the last cells to be disorganised when the other epidermal cells were collapsing (Fig. 2 h,j), whether on lamina or petiole.
In general therefore we may sum up the influence on plants of very short repeated exposures to the full light of a quartz mercury vapour lamp as follows:
Delayed germination; retarded growth and stunted habit of seedlings.
Partial inhibition of leaf development, especially lamina.
In some cases deeper green colour, and in one case loss of anthocyanin of lamina.
Promotion of leaf-fall with longer exposures and older plants.
Retardation of flower formation and opening of buds already formed.
On the other hand with exposures of 30 seconds or less indications of a different result have been obtained. In one case flowering seemed to be promoted (Pelargonium A.), in another the leaves of the irradiated plant ultimately surpassed those of the control in size (Pelargonium A.), and in a third case, after a month of illuminations for 30 seconds daily at a distance of 8 feet, there was a permanent and very favourable after effect (Trifolium).
INVESTIGATIONS CARRIED OUT BY A. WESTBROOK, M.Sc., AT BEDFORD COLLEGE, REGENT’S PARK
August-September, 1926.
C. Experiments with Voandzeia
Early in August 1926, seeds of Arachis1 and Voandzeia2 were germinated, two together in pots, in the cool greenhouse at Bedford College, the use of which was kindly granted by Professor Neilson Jones. Of these the Arachis seedlings nearly all damped off at an early stage but the Voandzeias were more successful and were used in most of the experiments. The plants were not attacked by any insects or fungi, and once the initial stages were over they grew healthily.
In both cases the soil used was a mixture of loam, leaf mould and builders’ sand in about equal parts. The seeds were only just covered with soil and were watered daily. The greenhouse was unheated (excepting for one week in September) but in sunny weather became very hot. The temperature was recorded by means of a Casella thermograph and varied considerably, the average maximum temperature being 90° F. and minimum 63° F.: the extremes noted were 50° and 105° F., with a maximum daily range of about 40° F. The temperature conditions were the same for both control and experimental plants and in both cases were probably favourable to their growth. The irradiations were performed in the cool laboratory adjoining the greenhouse (at about 60° F.), but were given at the hottest time of the day, between 12 and 2 p.m. (summer time). Any local heating-up effect of the lamp was much less than the difference in temperature between the two rooms; it was not determined, but in the light of previous observations made at Kew (p. 142) may presumably be disregarded. The relative humidity was recorded by means of a Casella hair-hygrograph; it varied generally inversely with the temperature but was partially controlled by watering the floor liberally in the morning, at midday and in the afternoon.
Seeds of Voandzeia, set on August 3 came up on August 14-16, the two first leaves expanded rapidly and the plants were illuminated under experimental conditions from August 18. On September 18 a number of small flower buds were visible on both control and artificially illuminated plants, but these neither opened nor set seed, although the experiments were not discontinued until October 5. This was perhaps due to the onset of unfavourable weather with little sun and a lower temperature.
Miss Ritson’s experience with regard to the time of flowering of Arachis grown at Kew suggested that it would be advisable to pay some attention to the length of daily exposure to sunlight (p. 141). Accordingly some of the plants were arranged in three lots of 8 pots each, and by means of artificial darkening or illumination with a 100 watt Ediswan gas-filled bulb 3 feet above the plants, the length of day was adjusted. The intensity of the extra illumination was thus not high, but according to American workers this is not important in experiments on photoperiodism (Garner and Allard 1920). The control and experimental seedlings in each group were selected so as to be comparable with each other at the beginning of the experiment. Four pots of each group were exposed to the Ulviarc lamp for short intervals daily (including Saturday and Sunday) at a distance of 3 feet from the plants. In all cases exposure was given after allowing an interval of 10 minutes for the lamp to reach its full intensity.
The controls were stood on a shelf facing south and given 12 hours’ daylight, beginning at first at 8 a.m. (summer time), later 7.30 a.m. and finally 7 a.m. as the days grew shorter. This alternated with a night of 12 hours, adjusted by covering the pots with black cloth arranged over a frame. The shorter days were adjusted in a similar way, thus:
The general results of the exposures may be briefly summarised under two aspects:
(1) The effect of varying the length of day (lots A, C, E) (Pl. III). The plants given a 12-hour day were much the best developed; they were taller, had more leaves and broader leaflets than the other two sets. The 5-hour day plants were lanky looking, they had only a few small leaves and the leaflets were a pale green; the 7-hour plants were intermediate in colour and general appearance. The root system was very well developed in the controls and was progressively weaker in the 7- and 5-hour day plants. The reduced synthetic powers of these plants is shown by the fact that, after 9 weeks’ growth, the cotyledons of the 5-hour day set had disappeared—those of the 7 hours were present, though shrivelled, while in the controls they were almost unchanged. This was also confirmed by estimations of dry weight and ash content of two plants of each series, the average dry weights diminishing in the ratio 1:0·55: 0·40 and the ash content 1: 0·62: 0·43 with progressively shortened day period (1: 0·58: 0·42).
(2) The effect of additional exposure to ultra-violet light (lots B, D, F) (Pl. III). The illumination given was 2 minutes daily at a distance of 3 feet: this was continued until October 7, when the experiment was stopped and the plants photographed (Pl. III). After 3 days’ treatment the leaflets were already somewhat reflexed, with their upper surface darker in colour than the controls, excepting where one leaf was shaded by another. In seedlings of set F, the leaflets had turned reddish brown in patches. On examination in October, after 7 weeks’ irradiation, it was found that plants from set B (12-hour day) had their leaflets modified: they were darker in colour, smaller in area, had brown and recurved tips and brown marginal patches, with their edges slightly upturned. The petioles also were markedly brittle, but there was no sign of actual leaf fall; the root system was about half the size of that of the control. Plants from set D showed the same features but more noticeably; they were somewhat stunted in size, while only a very few withered flower buds were present.
Seedlings of set F, with only a 5-hour day, were severely checked in development and were dead before the end of August; their remains were photographed on October 7 (Pl. III F).
For two plants from each of the sets B and D the fresh and dry weights and ash content were determined for comparison with A and C respectively. It was found that the irradiation resulted in a further diminution of the fresh and dry weights and of the ash content as shown in Table III.
Another series of experiments was carried out, using Voandzeia seedlings set on August 12 and illuminated with the Ulviarc lamp from August 23. These were subdivided into two groups (IV and V), each subgroup of which consisted of four pots, mostly with two plants in each. In some sets the natural daylight was supplemented with electric light at night to see whether the continuous illumination would influence the reaction to ultra-violet light. For this, as before mentioned, a 100-watt bulb was used at a distance of 3 feet. The sets were as follows:
The plants did not experience in their initial stages such hot weather as did sets A∼F and so they did not grow so fast nor did they have such broad leaflets. They all showed a slightly different habit in the formation of short prostrate branches bearing erect petioles. On September 14 flower buds were noticed on all of the plants: none of these opened or set seed, but on examination it was seen that stamens and carpels were well developed.
The condition of the plants on October 7 may be seen to some extent from the photographs then taken (Pl. III) and may be briefly summarised for the different series, each of which had received 45 days’ treatment.
In Group IV the controls (Pl. II H) receiving full daylight were tall and bushy, with marked development of prostrate branches, numerous flower buds and a well developed root system. Set ℱ (with 5 minutes u.v.l.) showed stunting, few prostrate branches, brittle tissues, dark green sickle-shaped leaves with upturned margins and brown patches round the veins on the upper surface; flower buds were present, but shrivelled, and the root system was very feebly developed. Set K (10 minutes u.v.l.) resembled set ℱ but the stunting was more marked, there were no prostrate branches and only a very few shrivelled flower buds, the leaflets were browned and more reflexed and the roots even less developed. In Group V.the effect of adding electric light at night was very slight: the leaves appeared more numerous and the leaflets narrower in the controls (G) than in the plants of set H, given a normal day and night. In set M (Pl. III) with only 1 minute u.v.l. daily the plants were shorter, the leaves deeper green with shiny patches, the leaflets narrower and shorter, flower buds were fairly numerous, there was a poorly developed root system. Set L (2 minutes u.v.l.) showed the same characteristics but more accentuated, and set I with 5 minutes u.v.l. closely resembled the stunted, wizened looking plants of set K which have already been described.
A number of plants (three or four from each set) were measured at the end of the experiments—the height, number of leaves, length of the petioles, and length and breadth of the leaflets being taken and averaged for each. In Fig. 3 the relative average dimensions for two characters are expressed graphically, taking 100 as the value for A (control to sets B–F) and H (control for sets G–M). These show the most clearly the stunting effect of exposure to ultra-violet light, but the same could also be deduced from the other characters mentioned. If these results are the direct expression of the reaction to the irradiations given, one would expect them to conform to the Bunsen-Roscoe law for photochemical reactions—that the Total effect produced = constant × intensity of illumination × time of illumination.
Actually this relationship does not hold for the characters considered, the values obtained for the constant varying widely. This may perhaps be explained by the length of the experimental period, since permanent changes induced early in the course of the experiments would be indistinguishable from those appearing later in groups receiving less illumination.
The general conclusions to be drawn from these experiments on Voandzeia are:
(1) That the extra illumination at night caused in normal plants a greater production of leaves but narrower and shorter leaflets; it had no appreciable effect on the response to additional treatment with ultra-violet light, nor did it promote; flowering
(2) That the results of irradiation with ultra-violet light confirmed those of the former series A–F in every way.
A few plants of Arachis were available for experiment. Three pots, set on August 13, were given from August 21:
(a) Full day and night in the greenhouse.
(b) Full day and night and 5 minutes u.v.l. daily.
(c) Full day and night and 10 minutes u.v.l. daily.
The controls flowered on September 30 and were still blooming a week later. Those given ultra-violet light were stunted, had fewer leaves, the leaflets being yellowish with upturned margins. The plants with 10 minutes daily illumination produced no flowers, but those with 5 minutes began flowering on the same day, although not so freely as the controls. No seed was set in any case. The condition of the plants on October 10, after 45 days of experiment, is shown in Pl. III R.
A systematic examination of the experimental plants is in progress and will be described in a separate communication. Measurements have been made of selected leaves from comparable positions on the main axis of different plants, and the anatomy of representative leaves from each of the sets A–E and G, H, I has been investigated by Miss M. Martin, B.Sc. The results already obtained may be briefly summarised.
(1) There is no appreciable anatomical difference between leaves receiving normal day and night and those subjected to continuous illumination, 12 hours daylight and 12 hours electric light (cp. Pl. III H, G).
(2) Either of these types of illumination combined with an exposure of 5 minutes daily to the Ulviarc lamp involved definite anatomical changes during development:
(a) Reduction in total thickness of lamina.
(b) Development of more compact mesophyll with smaller and fewer air spaces.
(c) Reduction in mechanical tissue.
(d) Collapse of cells of upper epidermis followed by the withdrawal of the chloroplasts from the upper parts of the palisade cells.
These points are illustrated in Fig. 4 for sets H and I.
(3) Plants receiving only 2 minutes ultra-violet light daily were mostly not very different from those receiving 5 minutes; in some of these plants areas with only partial collapse of the upper epidermis were noted: these presumably corresponded to the shaded regions which had been observed to be a different colour during life (p. 147); in these plants, unlike those of Trifolium (p. 145), the guard cells collapsed in advance of the other epidermal cells.
(4) Anatomical features of leaves which had different lengths of day (series A, C, E, p. 147) were alike, excepting for a progressive decrease in thickness.
(5) The effect of additional exposure of these plants to ultra-violet light showed in:
(a) A decrease in thickness in each case.
(b) More compact mesophyll and palisade tissue.
(c) Reduction of mechanical tissue.
(d) Complete collapse of upper epidermal cells.
These features were found in the plants which had received a day of 12 hours as well as those which had a 7-hour day, but those receiving a 5-hour day were already dead at the time of examination, and it was too late in the course of the experiment for observation of the progressive nature of the anatomical changes which had doubtless occurred in the three cases.
DISCUSSION
The experiments which have been described show the marked effect of irradiating plants for very short periods with the full light of a mercury vapour lamp. In the case of Voandzeia the external and internal changes have been observed, using daily intervals of irradiation of 10, 5, 2 and 1 minute respectively at a distance of 3 feet from the plants. The modifications observed were all in the same direction, differing only in degree. In Trifolium seedlings B, Table II, previously grown normally for 6 weeks, illumination for 30 seconds daily at a distance of 8 feet from the plant had a different result, and after one month (21 exposures, 5 days a week) the growth of the irradiated plants became more vigorous than that of the controls. There were only five experimental plants, but this effect resembles that found for Pelargonium at a distance of 3 feet (Nov. 1–29), see p. 143, and may have a real significance. We have no knowledge of the energy value of the emissions from the lamp, but if we call the total illumination for 10 minutes at 3 feet 1000, then the value for 30 seconds at 6 feet (Pelargonium) is 50, and that for 30 seconds at 8 feet (Trifolium B, Table II) is only 7. The other experiments at Kew were conducted, using short times of exposure, equivalent to 5 minutes or less, varying with the possible variations of the lamp at lighting up, and these can hardly be adequately estimated for intervals of less than 5 minutes.
The full radiations from the Ulviarc lamp includes bright lines in the blue, green and yellow, as well as many bright lines representing shorter wave-lengths than any found in the solar spectrum at the level of the earth’s surface. Further experiments are needed to determine the part of the spectrum which was the most active in bringing about the changes observed. From the work of Schanz and others, the presence of visible light in the spectrum would be expected to diminish rather than to increase the biological effect of the ultra-violet rays. When the exposures are given to plants receiving an artificially shortened day (7-hour, 5-hour) the resulting effect is much more marked than in those given a normal day. This may be due to the weaker plants produced by the shorter days and recalls the much greater effects found by Russell and Russell on exposing plants grown in the dark to ultra-violet light as compared with those grown in the light.
Structural changes have been briefly indicated for some of the experimental plants. A noteworthy feature is the early collapse of the epidermis in the plants irradiated daily for 2 minutes or more, forming in Voandzeia a firm, brown membrane, which appears to become cuticularised. This development may account for the distortion of the leaflets and the involution of their margins with subsequent growth and possibly also explains the compact nature of the mesophyll (Fig. 4). That the collapsed cells formed a real protection is indicated by the prolonged survival of the mesophyll cells immediately beneath them, and is in keeping with the observations of Kluyver (1911) and of Ursprung and Blum (1917), using longer exposures at a nearer distance given to well grown plants for 2–3 days only before examination.
REFERENCES
Legends for plates
In all Miss Ritson’s experiments the illuminations were given on only 5 days out of the week, Saturday and Sunday being omitted for convenience.
Hybrids from P. inquinans Sol. and P. zonale L’Hér.
Arachis hypogaea L., the ground- or peanut, indigenous to Brazil and Paraguay.
Voandzeia subterranea Thou., the Bambarra nut, indigenous to tropical Africa and Madagascar.