146 A TEXTBOOK OF THEORETICAL BOTANY 



walls are almost indestructible there are many beds of fossil diatoms known, 

 formed in ancient seas, which produce the fine powders used commercially 

 for polishing, called Diatomite or Kieselguhr. 

 There are two Orders in this group : — 



1. Centrales, in which the shape of the cell is similar to that of a 



circular pill-box. These are predominantly marine. 



2. Pennales, in which the cell is elongate and boat-shaped (naviculoid) 



in outline. They occur predominantly in fresh waters. 



As a type we shall select one of the latter, an uncommonly large form 

 which is very widely distributed in ponds and ditches. 



Pinnularia viridis 



Several closely related species such as Pinnularia nobilis and P. major are 

 also covered by the following description. 



The cells are elongate-elliptical in outline, seven to eight times as long 

 as broad and with rounded ends (Fig. 134). The usual size is about 20 X 140/x. 

 The siliceous wall or frustule is in two halves, the valves, corresponding to 

 a box and lid. The species is unique among Diatoms, in being enveloped 

 in a wide mucilaginous capsule. The view looking down on the lid is 

 called the valve view, and that from the side, where the margins of the 

 box and lid meet, is called the girdle view. From the side the cell looks 

 rectangular. Each valve itself consists of two closely united but separable 

 portions, i.e., the fiat valve face and the mantle, which forms part of the 

 girdle side of the cell. The mantles of the two valves do not overlap each 

 other but are connected by a separate girdle band (Fig. 137). 



Each valve face has a clear median strip running from end to end, in 

 which are three thickened spots, the central nodule, and two polar nodules, 

 connected by a very narrow slit, the raphe. On each side of the median 

 strip is a row of parallel transverse ribs, or costae, very prominently marked. 

 The girdle side of the frustule is unmarked except for the ends of the costae 

 which appear at its edges (Fig. 135). 



The costae are really hollow channels in the thickness of the frustule with 

 an opening inwards and containing pectin and cytoplasm. The raphe is a 

 very narrow slit, bent as shown in the section illustrated in Fig. 137, and 

 penetrating the cell wall, though there may not be a clear channel right 

 through. At the nodules it follows a twisted and very complex course. 

 Inside the siliceous coat, in the living cell, is an inner wall of pectin, within 

 which is a utricle of cytoplasm, surrounding a large central vacuole, and two 

 very large plastids, each as long as the cell and occupying the whole breadth 

 of the girdle on each side of the cell. In each plastid there is one pyrenoid. 

 The nucleus is centrally placed (Fig. 136). 



Reproduction 



When a diatom cell divides, nuclear division is followed by the separation 

 of the two valves and the cleavage of the cytoplasm into two parts. Each 



