ALG^E. 



441 



Dcsmidiece and Diatomea ; and among unicellular Algaa it has been ob- 

 served in the zoospores of Botrydium. 



In Spirogyra (tig. 512, A, a}, Zygnema, and Fiff. 505. 



one or two other genera of filamentous Confer- 

 voids, ordinary growth by cell-di\ ision is ex- 

 changed for a process of conjugation at certain 

 epochs. Two filaments, lying side by side (tig. 

 512, A, J), exhibit papillary elevations of the 

 cell-walls on the sides next their neighbours: 

 these processes elongate until they come into 

 contact; they then adhere, and the septum 

 formed at the plane of union becomes absorbed, 

 so that the two cells become connected by a 

 tubular process, a kind of isthmus. The contents 

 of the cells meanwhile retract themselves from 

 the wall, lose their spiral appearance, and be- 

 come condensed into a mass ; then, in some cases, 

 the whole contents of one cell travel through the 

 isthmus into the opposite cell (c) j in others, the 

 contents of both pass into the isthmus, which 

 expands into a globular cavity in the middle. 

 In either case the contents of the two cells be- 

 come combined, and they form a globular or oval 

 spore, which produces two or three firm coats, 

 enters a stage of rest, and remains after the pa- 

 rent filaments have decayed away (fig. 512, A, d). 

 After a time, usually in the spring succeeding the 

 formation of the spore, this germinates, bursting 

 its coats and sprouting out into a new filament 

 like the parent (tig. 512, A, e). This conjugation 

 of Spirogyra and its allies has long been known, 

 and was without a parallel for many years ; but, 

 as stated above, an analogous process occurs in 

 Diatomece and jDesmidiea and other plants, and it 

 is essentially related to the processes of fecunda- 

 tion by spermatozoids next to be described. 



Fecundation by Spermatozoids. The history 

 of the fertilization in CEdoyonium is one of the 

 most curious points in the whole range of vege- 

 table physiology, especially so as regards the 

 male organs, which undergo a complex course of 

 development as follows. On the same plant that 

 produces the female spore, or in some species on 

 another individual, are formed special cells 

 called " microgonidia " or " androspores." The 

 office of these cells is to produce ultimately an- 

 theridia, in which latter spermatozoids are 

 formed. The androspores are formed in the 

 ordinary cells of the plant, and escape from them 

 by rupture of the walls of the parent cell as an 

 oidinary zoospore would do, and like it they swim about in the water for 



(Edogonium ciliatum : A, ordi- 

 nary cells, in each of which 

 a zoospore (E) is formed; 

 C, C, sporangia; B, B, an- 

 drospores, one bearing at a 

 an antheridium, the lid of 

 which is detached; D, ex- 

 tremity of the plant. 



