108 



BOTANY 



Order IV. Conjugatae 



The Conjugate differ so much from the Chlorophyceae that have 

 just been considered that they are sometimes removed entirely from 

 the Green Algae. They differ most in their reproductive cells, which 

 are never ciliated, so that fertilization is usually effected by the 

 formation of a tube connecting the similar conjugating cells. The 

 plants are either unicellular or simple rows of uniform cells. 

 The cells are always uninucleate, the nucleus occupying the centre 

 of the cell, and connected with the peripheral cytoplasm by more 

 or less evident cytoplasnuc threads or bands. The chromatophores 



are large, and of various charac- 

 teristic forms, spiral bands, 

 flat plates, stellate masses, etc. 

 Small bright drops (tannin vesi- 

 cles) are characteristic of many 

 species. The cell-wall is usually 

 delicate, and the cells often 

 secrete a gelatinous matter, in 

 which they are imbedded, so that 

 masses of the plants are slimy 

 to the touch. 



The Conjugates are widespread 

 in fresh water, but never occur 

 in the sea. They may be divided 

 into two families, the Desmidi- 

 acese and Zygnemacese. A third 

 family, Mesocarpaceae, is some- 

 times recognized. 



The Desmidiaceae 



The Desmids are unicellular Algae, 

 often of exceedingly beautiful form. 

 The simplest (Mesotsenium, Spiro- 

 tsenia) are oblong or spindle-shaped 

 cells, with a delicate cell-wall. In the 

 former genus there is a single flat 

 chromatophore, which occupies the 

 long axis of the cell, and contains a 

 single pyrenoid. In Spirotsenia (Fig. 

 79, A) the chromatophore is a spiral 

 band applied to the inner surface of the cell-membrane. 



In most of the Desmids the cells show a marked bilateral symmetry. This 

 is indicated by a division of the cell-wall into two valves, one of which overlaps 

 the other, not unlike what is found in the Diatoms. The cell-contents also 

 show a corresponding symmetry. There are usually two chromatophores, one 



FIG. 79. A, Spirolxnia muscicola(x350). 

 (After DE BARY.) B, Penium inter- 

 ruptum (X 250) . C, Euastrum pingue 

 (X550). D, Closterium lunula; p, 

 pyrenoids; v, vacuole, containing gyp- 

 sum-crystals. E, Staurastrumgracile ; 

 I, from above ; //, from the side. The 

 chromatophores are shaded. 



