. A handbook of cryptogamic botany. Cryptogams. DIATOMACEM 421 \ 1 Fig. 'i'^^^nceneis spherophora. a^ c, girdle- view ; 6, valve-view. The endochrome-plates are shaded ( x goo). (After Pfitzer.) or zonal view; the aspect in which the surface of the. valve is turned to- wards the observer is the side or valve view. In many diatoms the central space on the valve view not occupied by transverse strise shows at its middle and at each end a strongly refractive thickening known as a node or nodule; and these nodules are connected with one another by a longitudinal Ime or rib—the raphe or sutu
. A handbook of cryptogamic botany. Cryptogams. DIATOMACEM 421 \ 1 Fig. 'i'^^^nceneis spherophora. a^ c, girdle- view ; 6, valve-view. The endochrome-plates are shaded ( x goo). (After Pfitzer.) or zonal view; the aspect in which the surface of the. valve is turned to- wards the observer is the side or valve view. In many diatoms the central space on the valve view not occupied by transverse strise shows at its middle and at each end a strongly refractive thickening known as a node or nodule; and these nodules are connected with one another by a longitudinal Ime or rib—the raphe or suture. The primary classification of the genera of di- atoms usually adopted depends on the presence or absence of this raphe. Each diatom-cell contains a nucleus and a nucleole. The chlorophyll occurs in the form of plates or bands arranged with more or less symmetry, and there are usually also drops of oil, especially when conjugation is about to take place. A very few di- atoms are green; but in the great majority of cases the colour of the chlorophyll is obscured by a characteristic brown pigment known as diatomin, readily soluble in alcohol, forming a brownish-yellow solution which is only slightly or not at all fluorescent. With concentrated sulphuric acid it assumes a beautiful blue-green colour. Petit (Brebissonia, 1879-80, p. 81) has very carefully in- vestigated the chemical and physical properties of the colouring matter of diatoms. He regards diatomin as a compound of chlorophyll and phycoxanthin, and as having a great analogy with the chlo- rophyll of the higher plants, the two spectra being very similar. Many of the solitary species of diatom, such as those belonging to. Fig, 354 b.— valves, side Gompkonenta constriciitni Ehrb. j, view, showing nucleus : gi^ g,,^ girdle- views ; y, transverse section through middle of cell, showing siliciiied cell-wall, one half overlapping the other ; ^ kf nucleus ; ^, dense protoplasm ; jf., £,„ girdle sutfaces (magniiied).
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