of the Fresh iva ter A I gee . 29 



filament, is particularly distinguished by the presence of a pecu- 

 liar and regular annulation of the enveloping membrane of the 

 cells, which would appear at a certain epoch to be intimately ad- 

 herent to the tissue of the cells themselves (PI. I. fig. 6). 



This anmdar disposition of the sheath of the cells does not 

 occupy its whole extent, but corresponds only to certain cells and 

 determinate portions of those cells ; the cells around which it is 

 disposed being those in which the spores are destined to be formed, 

 and the portion of these which it invests being the extremities 

 through which no endochrome passes from the contiguous cells 

 for the formation of the true spores, or rather perhaps sporangia. 



The number of annuli which correspond to each fructiferous 

 cell varies considerably according to the species, and is more con- 

 siderable in the long-celled species ; it would appear however to 

 be never less than two, or more than eight or ten. 



The use of this interesting structure is much more apparent 

 than that of the pro\dsion already noticed as belonging to one 

 section of the genus Zygnema, and admits of a most satisfactory 

 explanation, it being manifestly designed to afford an outlet to 

 the imprisoned spores, which it may be supposed to do in the 

 following way. As soon as the species has reached its maturity 

 and the spores have become perfected, the annuli, which are inti- 

 mately united to the cells, contract, most probably from the arrest 

 of. growth and diminution of vitality of the plant which occur to- 

 wards the completion of the process of reproduction, di'awing 

 along with them, and thus rupturing, the shghtly elastic mem- 

 brane of the cells (PI. I. fig. 7). 



Without some such beautiful and effectual provision, it ^\'ill be 

 evident, on reflection, that the spores would have to remain im- 

 mm-ed wdthin their narrow cells for an indefinite length of time, 

 even until, perhaps, their \itality had ceased and the cells had 

 become their coffins; for occupjdng, as the spores do, but a portion 

 of the space of the cells, and enveloped as they are in mem- 

 branes, they can themselves, of course, exert no inflvience in pro- 

 ducing the rupture of the walls of those cells. 



In all the Confervfe with ^hich I am acquainted, some special 

 means are provided for the escape of the spores or zoospores, their 

 liberation never being left to the sole agency of decomposition of 

 the tissue of these plants ; thus, in the majority of the branched 

 Confervje, and in the species of the genus ^phctroplea as well as 

 in many other Confervse, their liberation is effected by the rup- 

 ture of the cells in which they are contained, which rupture is 

 occasioned by the development of the zoospores while still inclosed 

 within the cells; in Conferva (Mia-ospora) glomerata a special 

 aperture exists for the escape of the zoospores at the period of re- 



