Chapter VI 



— 41 — 



The Plastids 



cells. These chloroplasts sometimes have the same form and di- 

 mensions as in the higher plants, but often these organelles are 

 greatly differentiated, of a complex structure and are present in 

 the cell only in very small numbers. Sometimes there is only one 

 to a cell. In that case it is voluminous and generally contains small 

 refractive bodies which are colorless, rounded, or angular, known 

 as pyrenoids (Fig. 12). At the surface of these grow the starch 

 grains which form a sort of crown about them. Because of their 

 complex structures the chloroplasts are often designated by a spe- 

 cial name, chromatophores, but we shall see that they correspond 

 to the small chloroplasts of phanerogam cells (Fig. 10). The life 

 history of these chloroplasts is not difficult to follow and SCHMITZ 

 (1882) showed that they are always transmitted from cell to cell by 

 division, quite like the nucleus. Stras- 

 BURGER was even able to follow the divi- 

 sion of the single chloroplast in living 

 Spirogyra during cell division. 



The chloroplasts in algae and bryo- 

 phytes:- The study of the plastids will be- 

 gin with the algae. In the Cyanophyceae, 

 which are the most primitive of the known 

 algae and whose cellular organization dif- 

 fers from that of the others, there are no 

 chloroplasts. Their cells contain a chro- 

 matic network, the central body, which 

 occupies the greater part of the cell and 

 which must be considered as a primitive 

 nucleus. Surrounding this is a thin pari- 

 etal zone of cytoplasm which contains 

 chlorophyll in a diffused state to which is 

 added a blue protein pigment, phycocya- 

 nin, and sometimes a red protein pigment, 

 phycoerythrin. Some authors have called 

 this parietal layer a chromatophore (Fischer, Meyer), but this 

 opinion seems difficult to justify, since the parietal zone does not in 

 the least resemble a chloroplast and encloses, all around the cen- 

 tral body, small vacuoles which can be stained in living tissue with 

 neutral red. As well as the Cyanophyceae, the algal Flagellates 

 (Phytoflagellaceae) are considered to be most primitive algae and 

 are thought to be the ancestors of all the other algae. In these, 

 the chloroplasts in each cell are sometimes numerous, as in many 

 Euglenas, and look like those found in the phanerogams. In other 

 Euglenas there is only one star-shaped chloroplast. In the Peri- 

 diniales, which are also classed among the flagellated algae, the 

 chloroplasts may be small, numerous, rod-shaped bodies or may be 

 reduced to a single body appearing as a fine network and occupying 

 the entire cell (Chatton). In many flagellated algae (Chrysomo- 

 nadales, Polyblepharidales), however, there is only one bell-shaped 

 chloroplast which occupies one of the poles of the cell, a position 



Fig. 11. — Motile cell of 

 Chlamydomonas surrounded by a 

 cell wall which forms a beak 

 between the flagella at the an- 

 terior pole. Stippled region 

 shows chloroplast. st, stigma. 

 p, pyrenoid surrounded by o, 

 starch, n, nucleus, v.p., pul- 

 sating vacuole whose role is 

 imperfectly known. (X 1650). 



