DIV. i THALLOPHYTA 389 



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 with 

 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 (Fig. 79) and one or two to 

 four (Fig. 323) large, or numerous smaller chromatophores (Fig. 320) 

 embedded in its parietal protoplasm. These chromatophores are flat, 

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

 often present. The pigments are chlorophyll and yellow phycoxanthin. 

 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 consequence of this peculiar manner of 

 division, since the walls of the cells are silicified and incapable of dis- 

 tension, 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 which they arise, and by 

 their further development they re-establish the original size of the cells. 



The sexual reproduction consists of a conjugation of similar gametes. 



The Diatoms include two orders, Centricae and Pewnatae. 



The auxospores in the Centricae, which are characterised by the centric structure 

 of their valves, grow from vegetative cells without any previous process of conjuga- 

 tion. In the Pennatae, with a pinnate sculpturing of their valves, on the other hand, 

 the auxospores develop from the zygotes resulting from a conjugation of gametes. 

 The Pennatae have diploid vegetative cells, the reduction division taking place 

 in the formation of the gametes. The Centricae are haploid and the simple 

 chromosome-number has been attained at the division of the zygote, which so far 

 as is known is in them the result of the fusion of ciliated gametes. The two 

 groups of the Diatoms are thus sharply distinguished. 



Order 1. Diatomeae Centricae 



In these the valves are symmetrical about a centre, and have the sculpturing 

 radially or concentrically arranged. The great majority of the forms of this order 

 are marine, and play a large part in the composition of the plankton ( 18 ). The 

 plankton diatoms are provided with special arrangements for floating, e.g. horn-like 

 projections or wings of the cell wall such as are seen in Figs. 320 and 321. 



2 Cl 



