4 TILE VOYAGE OF LI. M.S. CHALLENGER. 



The siliceous cell of the Diatom is filled with finely granular protoplasm, as may be 

 seen after treatment with water slightly acidulated with hydrochloric acid. This causes 

 the protoplasm to contract so that a vacant space appears along the sides of the frustule. 

 The protoplasm encloses a coloured substance called endochrome, as well as a few 

 round granules of an oily nature which dissolve under the action of ether or carbon 

 bisulphide. 



The endochrome is composed of chlorophyll and diatomine or phycoxanthine, 1 but of 

 these the former is the more interesting, as it is capable, under the influence of light, of 

 decomposing carbonic acid and evolving oxygen. The endochrome is of a greenish- 

 yellow colour, and occurs in the form of bands, granules, or rounded masses which may 

 be arranged irregularly or in radiating lines. 



The appearance which this endochrome assumes in the same species is not always 

 identical, and frequently, when it occurs as an amorphous mass, it becomes subsequently 

 divided into numerous granules of equal size and definite outline. That these are to be 

 regarded as so many perfect embryonal cells which are destined to reproduce the parent 

 form cannot be doubted (see pp. 7, 8). 



In addition to the endochrome and oleaginous substance, there is, in the centre of the 

 cells, a protoplasmic mass distinguishable by its density from the protoplasm which occupies 

 the rest of the interior of the Diatom. This is called the Cytoblast or Nucleus, and 

 in its turn encloses the Nucleolus in its inactive condition, as may be demonstrated by 

 the action of a very weak solution of chloride of gold. 



The cytoblast and the nucleolus certainly exist in all Diatoms, but it is not very easy 

 to distinguish them in small forms. In large species, however, such as Striatella unvpunc- 

 tata and a few Amphora, these structures may be readfly seen, and in these the cytoblast 

 may be observed to possess a stellate form having filaments resembling the pseudo- 

 podia of Diffluc/ia or other Infusoria which sometimes extend to the extremities of the 

 frustule. 



Diatoms are solitary or gregarious, free or fixed. The fixed forms are attached by 

 means of a peduncle which may be simple or branched, or they may be enclosed in a cyst, 

 fastened on a small cushion, or embedded in a gelatinous substance which forms a frond 

 either in the form of a simple or compound tube, of a flattened plate, or globular mass. 

 In seriate species the frustules may be arranged either in a lineal or lateral manner. They 

 are also often met with in the form of zigzag chains, adhering to each other by means of 

 small angular cushions, as in Tabellaria. 



The substance which exists in the form of a definite frond resembling a higher alga, 

 being either ramified, ulvaceous, tubular, saccate, or mucilaginous and amorphous, and 



1 Sachs' Text-Book of Botany ; Millardet and Kraus, Compt. Rend., vol. Ixvi. p. 505 ; Askenasy, Botan. 

 Zeit., 18G9, p. 799; Hitzer in Heft, ii. Botan. Abhandl., edited by Ilanstein, Bonn, 1871 ; Quart. Jour. Micr. 

 Sex, 1872-73. 



