264 Bacillariece 



pigment known as diatomin, which resembles the phycophsein of 

 the Phaeophyceae. The diatomin, which can be extracted by 

 alcohol forming a yellow-brown solution, is itself a complex sub- 

 stance containing, amongst other pigments, xanthophyll. An 

 alcoholic solution of diatomin turns a beautiful blue-green colour 

 on the addition of sulphuric acid. The chromatophores contain a 

 variable number of pyrenoids which often project into the interior 

 of the cell as rounded elevations. Mereschkowsky 1 has observed 

 pyrenoids which have partially or entirely emerged from the 

 chromatophores, appearing as free colourless bodies on their inner 

 surfaces. 



The nutrition of the vast majority of Diatoms is holophytic, 

 but a few saprophytic forms are known' 2 . The latter are peculiar 

 in the complete absence of pigment, and they apparently occur 

 in water in which there is an abundance of decaying organic 

 matter 3 . Karsten 4 has found that Nitzschia palea (Kiitz.) W.Sm. 

 when cultivated in favourable nutritive media will become 

 saprophytic. 



Diatoms are incapable of growth in size owing to the siliceous 

 nature of their cell-walls, but slight alterations of volume can take 

 place by a sliding movement of the connecting-band of the older 

 half of the cell over that of the younger half. 



In addition to the symmetrically arranged markings on the 

 valves, the frustules of Diatoms possess an external symmetry in 

 one or more planes. Some of them are zygomorphic in one plane 

 only, some in three planes at right angles, and others exhibit a 

 radial symmetry. 



The movements of Diatoms : Most of the solitary, unattached 

 species of Diatoms exhibit movements which have long been a 

 puzzle to students of biology. This power of locomotion is 

 especially marked in species of a naviculoid form, and various 

 explanations have at different times been put forward to account 

 for it. The movements of some forms are very slow, but others 

 are capable of propelling themselves with considerable rapidity 

 backwards and forwards in the direction of their longer axis. This 

 spontaneous movement is sometimes creeping and steady, but at 



1 Mereschkowsky in Flora, xcii, 1903, pp. 77 83. 



2 The following are colourless saprophytic Diatoms : Nitzschia putrida Benecke, 

 N. leucosigma Benecke, and Synedra hyalina Provasck. 



3 Benecke in Pringsheim's Jahrb. f. wissensch. Bot. xxxv, 1900. 



4 Karsten in Flora, Ixxxix, 1901. 



