THE ALGJE 



111 



may usually be seen numerous starch-granules. The large nucleus is suspended 

 in the centre of the central vacuole by protoplasmic filaments attached to the 

 pyrenoids. Cell-division may take place in any cell, and occurs normally at 

 night. After the nucleus divides, the protoplast is cut in two by a ring-shaped 

 wall, formed about the equator of the cell, and growing inward until the division 

 is complete (Fig. 81, U). 



Conjugation. Conjugation in the Zygnemacese is very much like that in the 

 Desmids (Fig. 81, A-C). From neighboring cells, either in the same filament 

 or an adjoining one, protuberances are sent out which, fusing together, connect 

 the cells. The protoplast may leave both cells and unite in the conjugating 

 canal, or, as in most species of Spirogyra, 

 one of the gametes remains within the cell, 

 and the other passes through the canal to 

 it. There is usually a contraction of the 

 cell-contents preliminary to their fusion, 

 but in the Mesocarpacese there is little or 

 no contraction of the cell-contents, recalling 

 in this respect the behavior of the very 

 similar Mesotsenium. 



As a rule, the fusion of the nuclei, so far 

 as it has been studied, occurs soon after 

 the fusion of the gametes. A fusion of the 

 chromatophores has also been demonstrated 

 in Spirogyra. The ripe zygospore loses 

 its chlorophyll, and generally assumes a 

 dark brown color, and its contents appear 

 coarsely granular, owing to the accumula- 

 tion of reserve food matter. 



The spores may retain their vitality 

 for several years. On being placed FIG. 82. Germination in Spirogyra 



lonyata (?). 



x ioo. 



A, B X 250. C 



in water they quickly absorb water, 

 and within a week or so begin to 

 show signs of germination. The green color, is restored, and growth 

 begins. The outer membrane is ruptured, and the young plant 

 pushes through the aperture. In Spirogyra (Fig. 82) the elongated 

 primary cell tapers at the base, which is enclosed within the spore- 

 membrane. Large drops of oil are seen, which gradually are used 

 up as the young plant develops. 



Occasionally Aplanospores (Fig. 83, D-E) are found, which closely 

 resemble the zygospores except that they arise without fecunda- 

 tion. 



Affinities of Conjugatae. The lower Desmids, both in their 

 structure and reproduction, recall the simpler Volvocacese and Pro- 

 tococcacese, and are probably related to them. From these simpler 

 Desmids the more specialized forms are easily derived. 



The structure of the cell-wall, as well as the reproduction, sug- 

 gests the Peridineae and Diatoms, which may also be remotely 

 related to the Desmids. The resemblances, however, are probably 

 only analogies. 



