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10 



MISS B. M, BRISTOL ON 



63 X 165^, while one pear-shaped cell in which the division of the contents 

 was observed measured 105 /x. broad and 178 yx long. 



The cell-wall consists of cellulose, and is frequently not more than 2 /a 

 thickj though it may increase at the time of zoogonidia-formation to about 

 3'5 fc^ when it shows a few faint striations. It usually bears on its outer 

 surface one or more finely-striated projections formed by the unequal 

 thickening of the cell-wall^ though in some individuals no such projections 

 were ever produced even at maturity. The form and extent of the 



2. figs. 17-23) 



Ai 



s 



a general rule the inner surface of the cell-wall bears no special thickenings, 

 but in a few cells such as those shown in figs. J 7 and 18 internal projections 

 were present. These, however, were never found to be of any great size and 

 were observed only rarely^ but their occasional presence shows to what a 

 considerable extent the development of the cell-wall is liable to individual 

 variation. 



The liviiif;- cells differ in appearance according to their state of develop- 

 ment. Young cells and others which have grown qnickly show quite clearly 

 that the protoplasm is differentiated into two parts— a colourless part and a 



chloroplast. An optical 



&27) 



that the chloroplast is of the same type as that described for C. Lemnw, 

 though the radiating branches ate somewhat more robust, and the parietal 

 portions are fre(|uently less expanded and rarely fused together ; in many 

 cases a surface view gives the impression that the cell contains a number 

 of discoid parietal chloroplasts (tig. 26). As the cell approaches maturity 

 the radiating branches seem to increase in number^ so that the cell-contents 

 assunui a very much darker green colour, and the internal structure becomes 

 very difiicult to make oat from the living material. Eventually the cell 

 appears to be homogeneously green. 



That this is not true, however, is seen from stained sections of the cells, 

 cut in paraffin after fixation with Flemming's weaker solution. The sections 



were 4-8^ thick, and were stained with Delafield's htematoxylin or with 

 Heidenhain's iron-aluui-hreraatoxylin. In sections prepared in this way the 

 nucleus is seen to be displaced a little from the centre of the cell, its place 

 being taken by a large pyrenoid. It is evidently this pyrenoid that produces 

 the clear space so noticeable in the centre of the chloroplast of some living 

 cells (fig. 25). The pyrenoid lias a large irregular-shaped pyrenocrystal, and 

 a number of very distinct starch-plates forming a somewhat irregular starch- 

 sheath. In a mature cell such as that represented in figs. 28-30 the chloro- 

 plast is seen to be massive, consisting of a small axial portion with repeatedly 

 branched arms radiating out from the central pyrenoid and practically 

 filling the whole cell ; within the cytoplasm of the chloroplast a very fine 

 reticulum can clearly b^ made out. The branches of the chloroplast are 



