162 FRESH-WATER A L GiE OP THE UNITED STATES. 



instead of a connecting tube being formed as the medium of union, two cells of 

 opposing filaments become sharply bent backwards, so that their central portions 

 are strongly thrust forward as obtuse points, which, coming in contact, adhere and 

 allow of a passage-way between the cells being made by the absorption of their 

 cohering walls. 



A curious modification of, or departure from, the ordinary method of conjugation 

 is sometimes seen, in the union of three instead of two cells. This is, I think, 

 very rare, but has been seen by Meyen in the genus Zy(jnema, as well as by 

 Schleiden and De Bary in Spirogyra. I myself have observed it once or twice in 

 the latter genus. One of the cells plays the part of the female, receiving the con- 

 tents of the other two, so that in it the primordial utricles of the three, with their 

 contracted protoplasm, are fused into a zygospore. 



The zygospore, however formed, varies in shape, but is mostly oval or globular, 

 sometimes cylindrical, and when ripe is in most if not all species of a dark brown- 

 ish color. It is described both by Pringsheim and De Bary as having three coats, 

 but I have frequently found it impossible to demonstrate the presence of all of 

 these, and I believe that not rarely one of them is absent. The outer coat is 

 developed first and is the thickest and firmest. Occasionally it is double, *. e. 

 composed of two distinct layers or parts, as in Sp. protecta, in which species the 

 outer of these layers is the thickest, firmest, and most evident, whilst the inner 

 layer is translucent and much less apparent. The second coat contains the 

 coloring matter, which is sometimes brown, sometimes decidedly yellowish. The 

 inner coat is not readily seen. It is elastic, thin, and is the last of the three to be 

 formed. 



The principal contents of the ripe spore are protein compounds (protoplasm), 

 oil-drops, starch granules, and pigment. The oil is generally much more abun- 

 dant than the starch, and not rarely the minute, bright drops entirely replace 

 the little granules. According to Prof. De Bary, the pigment frequently, but not 

 always, reacts with sulphuric acid, as does that of the fungal family, Uredinece, 

 striking with it a deep blue. 



The germination of the spore, both in the genus Spirogyra and Zygnema, is 

 very simple. The first step is an elongation and growth of the protoplasmic 

 central mass, together with the inner transparent cellulose coat, and a consequent 

 rupturing of the outer two coats, through which the newly forming plant protrudes 

 and finally escapes. In this way in the genus Spirogyra an elongated club-shaped 

 cell arises, one end of which is much larger than the other and contains all the 

 chlorophyl. Sometimes a nucleus is perceptible in this cell, sometimes it is not. 

 The larger end now becomes cut off by a partition wall from the smaller ; if no 

 nucleus has been previously apparent it now becomes so, and the first stage of 

 development is completed. The filament after this grows by a simple repetition 

 of the process of division in the larger end and the cells formed out of it. The 

 smaller end undergoes little or no change. In the genus Zygnema, the cell that 

 first emerges from the germinating spore is a perfect one, similar in all respects to 

 those seen in the fully formed filament, which is .developed out of it, by a simple 

 process of cell division. 



