MORPHOLOGY AND DEVELOPMENT 



THE CELL 



The cell membrane is composed of two usually equal parts, each of which consists of a 

 valve and a girdle or zone formed of cellulose modified by silica deposited in an insoluble state 

 from a very dilute aqueous solution. The valves are more siliceous and robust than the girdle. 

 Both are in most species easily separable, or at least the bands of the girdle which may be more 

 or less closely fastened to the valves have a motion over each other permitting the cell to en- 

 large at pleasure. The longitudinal diameter of the cell, or the distance between the centres 

 of the two valves, will vary according to the convexity of the valve and the age of the frustule 

 which may be often determined by the width or number of the girdle bands. These, owing to 

 their diversity of form and arrangement, will be further described under the generic diagnoses. 



The siliceous cell-wall is covered on the outside by a layer of protoplasm called the coleo- 

 derm. This layer may be quite thin and evident only when treated with fuchsin or Bismarck 

 brown, or it may be of considerable thickness. The cell contains the cytoplasma, protoplasm, 

 cell-sap, endochrome, pyrenoids, oil globules and nucleus, together with certain other less 

 understood bodies. 



The Cytoplasma is a thin skin of colorless plasma covering the entire inner surface 

 of the cell. It is invisible in the living cell but is evident in plasmolysis. In long forms 

 it is thickened at the ends and is condensed at the plasma bridge which frequently con- 

 nects the two valves and divides the cell into two parts, each containing more or less pro- 

 toplasm surrounding the vacuole in which are found the cell-sap and certain granules. 

 In some forms, as Meloseira, the cytoplasma includes the entire mass of protoplasm. 



The Endochrome is seen in the form of one or more bands or plates, of a yellowish 

 or brownish color, on the inner side of the valves or connective zone, or in granules or 

 irregular masses, more or less numerous, on the inner walls, or sometimes grouped near 

 the centre. It consists of a mixture of chlorophyll and diatomine which differ in their 

 relative solubility in alcohol and in their spectroscopic analyses. The color varies from 

 green to a chocolate brown in proportion to the amount of diatomine. So far as the func- 

 tion of the endochrome is concerned it does not appear to differ from that of ordinary 

 chlorophyll, absorbing, under the influence of light, the carbon, and disengaging the oxygen 

 of the carbonic anhydride in the water. Diatoms do not live in absolutely pure or non- 

 aerated water. The individual plates or granules of the endochrome are called chromato- 

 phores. Their number and significance will be referred to in the description of genera. 



THE PYRENOIDS. In the chromatophores of many species are found colorless, homo- 

 geneous bodies, strongly refractive, of various shapes, usually lenticular or fusiform, 

 which are known as Pyrenoids (Schmitz). They are scarcely evident in the living cell, 

 but are distinguished by the action of hsematoxylin and other reagents. Flat forms occur 

 in Surirella and Pleurosigma, lens forms in Pinnularia, Stauroneis, Synedra, Fragilaria 

 and Nitzschia, while a spherical form is found in Cymbella cuspidata. The pyrenoids 

 are always imbedded in the chromatophore. Their growth is by division. Schmitz con- 

 siders them a part of the living chromatophore, and their substance as working material 

 which in excess has become resolved into the nature of a crystal which its form sometimes re- 

 sembles. Comparisons are made between them and crystalloids found in certain monocoty- 

 ledons. The pyrenoid is evidently concerned in the formation of the chromatophore, or in its 

 division. Much of the conjecture, however, is due to the behavior of pyrenoids in other plants. 



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