Ultrastructural changes of chloroplasts and mitochondria have been observed in synchronously growing cells of Euglena gracilis Z, under photoautotrophic conditions. Application of the serial section technique allows estimation of the number and volume of these organelles. Several 3-dimensional reconstructions reveal their shape and distribution throughout the cell cycle. In young cells 10 separate diskoid or branched chloroplasts are found. They show the typical lamellar structure of euglenoid chloroplasts. During the growth phase (light period), they enlarge and their volume doubles. Some of them branch out, so that 20 lobes are formed. Thylakoids grow longer without change in number. The pyrenoid persists only during the first half of this period. During the cell division phase (dark period), branched chloroplasts divide along 2 planes which are perpendicular to each other and perpendicular to the thylakoid plane. All thylakoids are cut and their number does not change in the daughter chloroplasts. The plastidome volume constitutes 15–18% of the total cell volume over the entire life cycle. One of the most significant observations in this report is the presence of a single permanent mitochondrial reticulum during the whole cell cycle. This giant mitochondrion consists of an extremely branched network with delicate threads (0.4-0.6 micrometer thick) surrounding the chloroplasts, nucleus and reservoir. It extends throughout the cell. During the growth phase, it becomes gradually longer and doubles in volume. The degree of branching increases but the thickness of the threads remains constant. During the division phase, the mitochondrial elements appear more restricted (0.4 micrometer thick) and the reticulum becomes progressively partitioned into 2 daughter networks. At any time of the cell cycle, the chondriome volume is about 6% of the total cell volume. These results are discussed in comparison with numerous relevant papers on light and electron microscopy of animal and plant cells. They suggest that the descriptions of several authors on the plastidial cycle and the mitochondrial cycle in Euglena, both said to be characterized by alternate reticulate and fragmentary states, arise in part from questionable interpretation of random sections. It is evident that the form and distribution of organelles can be determined more precisely by serial sectioning.

This content is only available via PDF.