Cell-division 



387 



there arc no pyrenoids, but large numbers of small starch grains (fig. 240 B), 

 and in the autumn the cells of many species of (Edogonium are frequently 

 packed with moderately large starch-grains. 



Many of the cells in a filament 

 of (Edogonium may exhibit trans- 

 verse striations at their upper 

 extremities. This feature, which 



B 



Fig. 241. Three stages in the cell-division of 

 (EdofjonhiHi Borisianum (Le Cl.) Wittr. x about 

 620 (after Him). 



is most obvious in the larger 

 species, is due to a succession of 

 ' apical caps ' resulting from re- 

 peated cell-division (fig. 240 D). 



Growth is not apical, but for the 

 most part intercalary, and any cell 

 except the basal one may divide. 

 The details of cell-division have 

 been worked out by various authors 

 and notably in recent years by Hirn 

 ('00) and Kraskovits ('05). It is 

 preceded by the division of the 

 nucleus, during the mitosis of 

 which there is formed an intra- 

 nuclear spindle. At this period there arises a ring-like thickening on the 

 inner side of the wall at the apex of the cell (fig. 24>1 A). This ring is 

 a circular cushion, the central portion of which consists either of mucilaginous 

 material (Hirn) or of cell- wall substance containing a greater amount of water 

 than the ordinary cell-membrane (Wille), while the peripheral layer (towards 

 the interior of the cell) consists of cellulose. The latter becomes intimately 

 concrescent with the old wall above and below the ring. The first rupture is 

 probably that of the cuticle, which is torn irregularly (fig. 241 B), after which 

 the old cell-wall undergoes a circular split and the peripheral layer of the 

 ring becomes the new intercalary piece of cell- wall (fig. 241 B and G). The 

 new transverse wall arises as a cell-plate between the daughter nuclei and 

 gradually extends outwards to the old wall. This is similar to the condition 

 in higher plants and quite different from the in-creeping transverse walls in 

 Spirogyra or Cladophora. The upper part of the old wall is pushed upwards 

 as an ' apical cap ' by the gradual extension of the new intercalary wall and 

 the number of ' apical caps ' indicates the number of divisions any cell has 

 undergone. It is not improbable that the cuticle of the new piece of cell- 

 wall is derived from the material which formed the central part of the ring- 

 thickening. 



The plants of Bulboch&te are branched (fig. 242), often very profusely, and 

 are invariably attached. 



252 



