96 PRESU-WATER ALG^E OF THE UNITED STATES, 



There arc generally one or more hyaline spaces, besides a distinct chlorophyl 

 vesicle, but no distinct nuclei. 



At certain periods of their existence the Pediastrums produce both macrogonidia 

 and wicrogoiddia. The life-history of the former is very similar to that of the 

 same bodies in the water-nets. The ultimate fate of the microgonidia has not as 

 yet been determined, but in all probability they go through cycles of change 

 similar to those seen in the lives of the corresponding bodies in the Hydrodictyon 

 utriculatum. I have not had an opportunity of watching the development of 

 either of these reproductive forms, but, according to MM. Braun, Pringsheim, &c., 

 their life-history, as far as known, is as follows: In most cases, all the cells of a 

 pediastrum produce their macrogonidia simultaneously, or within a very short 

 period of time, so that the coenobium will be left emptied of its contents as a mere 

 shell, the outer skeleton of its former self. When a cell is about to give birth to 

 these reproductive bodies, the endochrome divides into two parts ; each of which 

 then undergoes a similar binary division. This is repeated once, twice, thrice, or 

 oftener, until the endochrome is divided into 8-16-32-64 gonidial masses, the 

 number of which, generally, but not always, corresponds to the number of cells in 

 the colony, to wliicli the parent-cell belongs. After the division of the endochrome 

 is completed, a slit occurs in the outer strata of the wall of the mother-cell through 

 wliich a hernial protrusion of the inmost stratum occurs. The protruded part 

 now rapidly enlarges until at last there is formed a sort of hourglass-shaped sac, 

 one portion of which is within, the other part without, the old paa'ent-cell. 

 Whilst this has been going on a portion of the gonidia have escaped from the 

 parent-cell into the outer free portion of the sac, and each end of tho hourglass, 

 therefore, contains some of them. The sac with its contents now gradually 

 withdraws itself more and more from the parent-cell until at last it lies a free 

 globose vesicle in the water. The gonidia occupy the centre, and M. Braun states, 

 that, although he has never been able to demonstrate any cilia upon them, yet 

 they have an active swarming motion. At first, they are irregularly heaped toge- 

 ther in the nearly filled sac ; but the latter rapidly enlarges and elongates, and the 

 gonidia in a little while arrange themselves in a flat, tabular group within it, and 

 cease to move. Then the several individuals of this group begin to develop, 

 becoming emarginate and assuming the form of the parent-cell, until, finally, they 

 have all grown into the shape which is peculiar to the adult cells of the species, 

 and after a few hours have closely cohered to form a young camobium. 



The microgonidia are formed in a very similar way by the dividing of the endo- 

 chrome, the cracking of the outer membrane, and the protrusion and final escape 

 of the inner. They are, however, much smaller and more numerous than the 

 macrogonidia. When the parent vesicle first escapes into the water, they are 

 crowded in its centre, and are nearly globose. As it enlarges, however, they elon- 

 gate more and more, and finally become distinctly bi- or, more rarely, uni-ciliate. 

 The cilia are much longer than the body, and are attached to the smaller end, 

 which is prolonged into a pointed, transparent beak, about equal to the green por- 

 tion in length. The microgonidia now become more and more restless, they, moving 

 about very actively, and after awhile bursting the parent sac, escape into the water. 



