CRYPTOGAMS 



349 



markings or ribs, and also with small protuberances and pits, or 

 they are perforated by open pores which serve to give exit to the 

 gelatinous secretion and possibly play a part in the movements 

 sometimes exhibited by the cells. 



According to the structure of the cell walls two main groups of Diatoms are 

 distinguished, the Centricae (Fig. 277) with usually circular valves and radial or 

 concentric sculpturing, and the Pennatac the valves of which are usually elongated 

 and the sculpturing pinnate. In many of the Pennatae (Fig. 3) a longitudinal 

 line corresponding to an opening in the cell walls, and exhibiting swollen nodules 

 at both extremities and in the middle, is distinguishable in the surface of the 

 valves. Forms provided 



with such a median suture XS^ZZTx. /"'"~" ^"v 7 " .^TIT^. l/b 



or i: A THE are characterised ^^ /^fes. A '-//*. 



by peculiar backward-creep- 

 ing movements, resulting 

 from the extrusion of pro- 

 toplasmic protrusions from 

 the spirally arranged longi- 

 tudinal slits which are pre- 

 sent in the raphe (cf. p. 266). 



The cell has always 

 a central nucleus and 

 one (Fig. 3) or two to 

 four (Fig. 275) large or 

 numerous smaller chro- 

 matophores (Fig. 277) 

 embedded in its parietal 

 protoplasm. These 

 chromatophores are flat, 

 frequently lobed, and 



of a brOWnish-yelloW I-'ic. STO. Coccone'is yliuxutulv. 1, Vegetative cell ; ..'. pair of 

 colour Globules Of a ce " s ' M '' ore conjugation ; J, , > t , cells in process of conjugation. 

 ' . ijl,-, r.art;e nucleus; /,/,-, small nucleus; 11, gelatinous sub- 

 tatty Oil are alSO 111- stance. (Alter KAHSTKN. from OI.TMASNS' .!?</" .) 



eluded in the cell con- 

 tents, and take the place of starch as an assimilation product. 



The Diatomeae multiply vegetatively by longitudinal division which 

 always takes place in one direction. In this process the two valves are 

 first pushed apart from one another by the increasing protoplasmic 

 contents of the mother cell, which then divides longitudinally in 

 such a direction that each of the two new cells retains one valve of the 

 mother cell. After the division of the protoplasm of the mother cell 

 is accomplished, each daughter cell forms, on its naked side, a new 

 valve fitting into the old one. The two valves of a cell are therefore 

 of different ages. In consequence of this peculiar manner of division, 

 as the walls of the cells are silicified and incapable of distension, the 

 daughter cells become successively smaller and smaller, until finally, 

 after becoming reduced to a definite minimum size, they undergo 



