SECT. I CRYPTOGAMS 351 



gelatinous branching thallus, often over 1 dcm. in size. The cells 

 also display a great diversity of shape ; while generally bilaterally 

 symmetrical, they may be circular or elliptical, rod- or wedge-shaped, 

 curved or straiglit. The structure of their cell walls is especially 

 characteristic ; it is composed of two halves or valves, one of which 

 overlaps the other like the lid of a box (Fig. 3, B). The cells thus 

 pi'esent two altogether different views, according to the position in 

 which they are observed, whether from the girdle (Fig. 3, B) or 

 VALVE SIDE (Fig. 3, A). The two valves are so strongly impregnated 

 with silica, that, even when subjected to intense heat, they remain 

 as a siliceous skeleton, retaining the original form and markings of 

 the cell walls. The walls of the cells, particularly on the valve 

 side, are often ornamented Avith numerous fine, transverse markings 

 or ribs, and also with small protuberances and pits. They are often 

 perforated by open pores which serve to give exit to the gelatinous 

 secretion. 



The cell has always a central nucleus and one (Fig. 3) or two to 

 four (Fig. 265) large, or numerous smaller chromatophores (Fig. 267) 

 embedded in its parietal protoplasm. These chromatophores are flat, 

 frequently lobed, and of a brownish -yellow colour. Pyrenoids are 

 often present. Globules of a fatty oil are also included in the cell 

 contents, 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 longi- 

 tudinally 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 consecjuence 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 transformation into auxospores. The auxospores 

 are usually two or three times larger than the cells from Avhich they 

 arise, and by their further development they re-establish the original 

 size of the cells. 



According to the structure of the cell walls and the mode of reproduction the 

 Diatomeae are subdivided into the two orders of the Pennatae and the Centricae. 



Order 1. Diatomeae Pennatae. — In shajie these are elongated, elliptical, or 

 boat-shaped, but may be wedge -slia[ied and zygomorphic ; the valves have their 

 sculpturing pinnate (Figs. 264, 265). In many of the Pennatae (Fig. 3) a 

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

 swollen nodules at both extremides and in the middle, is distinguishable in the 

 surface of tlie valves. Forms provided with such a median suture or kaphe are 



