Chlorophycece 53 



times they stand out clearly, occupying only a relatively small 

 portion of the cytoplasm. They may be solitary or very numerous, 

 of infinite variety of form, central or parietal, and the edges may be 

 entire or deeply incised. In some forms they are ribbon-like and 

 wound spirally round the interior of the cell-wall (e.g. Spirogyra, 

 Genicularia, and some species of Spirotcenia), and in others they 

 are central, spirally twisted masses (e.g. some species of Spirotcenia). 

 Sometimes they are reduced and very pale in colour, and in the 

 rhizoids and terminal cells of the hairs of some green Algae they 

 are entirely wanting. 



The chloroplasts of most green Algae contain pyrenoids or 

 proteid bodies which serve as a reserve of food-material. Much 

 has been done towards the investigation of these bodies during 

 the past few years and the presence or absence of pyrenoids has 

 been regarded by some as a sufficient generic distinction. This is, 

 however, attaching an importance to these proteid bodies which is 

 scarcely borne out by facts. Although they frequently divide 

 equally on the division of the cell, they also multiply without any 

 cell-increase. They are likewise known to disappear during the 

 development of certain species, and it has been clearly demon- 

 strated that during certain stages of Tetraspora, Sphcerella and 

 Eudorina they can arise spontaneously. Moreover, forms of An- 

 kistrodesmus falcatus (Corda) Ralfs containing pyrenoids are 

 sometimes met with in the same collection as others which have 

 no pyrenoids. Similarly, the chromatophores of Debarya calospora 

 (Palla) W. & G. S. West may or may not contain pyrenoids 1 . 

 Starvation causes a disappearance of pyrenoids and they frequently 

 increase in numbers if the cell is well nourished. On the whole, 

 there is little . doubt that the presence or absence of pyrenoids 

 depends largely upon external conditions and is a character to 

 which a great deal too much importance has been attached in 

 discriminating between the genera of green Algae. 



In the Chlorophyceae the stored product of assimilation is almost 

 invariably starch. Exceptions to this are found commonly in 

 Mesotcenium in the Desmidiacese and in the Vaucheriaceae. 



Cell-division generally takes place in all the cells of the 

 thallus, but in a few instances there is a definite growing point 

 which is usually an apical cell. 



1 West & G. S. West in Ann. Bot. xx, March, 1898, p. 49; in Journ. Bot. 

 Aug. 1900, p. 289. 



