CETPTOGAMOUS OR TLOWKELESS PLANTS. 



333 



and is now a spore, which when it grows begins a new series of in- 

 dividuals developed by successive division. 



656. In AlgiE consisting of a Single Row of Cells one tribe presents 

 the same mode of reproduction, and the various species of Zygnema 

 or Spirogyra, found in almost every pool of fresh 

 water at different times in spring and summer, 

 afford the readiest illustrations of conjugation, 

 which low powers of the microscope suffice to 

 exhibit. These green threads when magnified 

 are seen to consist of single rows of cylindrical 

 cells joined end to end. The cells being all 

 alike and equally capable of conjugation, each is 

 as it were an individual. At a certain season, 

 a protuberance appears on the corresponding 

 parts of certain cells of two adjacent threads; 

 the budding growth continues until the two 

 come into contact; the intervening walls are 

 then absorbed, opening a free communication 

 between the cavities of the two cells ; mean- 

 while the green matter and protoplasm, before 

 arranged in some definite shape in each species 

 (more commonly in one or more spiral bands), 

 break up into a granular mass floating in the 

 Water of the cell ; this all passes over from one 

 cell to the other, — sometimes to the one plaiit and sometimes to the 

 other in adjacent cells, — and is mingled with the similar contents of 

 the cell which receives it ; and the united product is condensed into 

 a green protoplasmic mass, which, acquiring a coat of cellulose, be- 

 comes a new cell or spore, in due time germinating into a new plant. 



657. In reproduction by conjugation, the two cells or individuals 

 concerned are alike ; one is as much the fertilizer or the fertilized 

 as the other. But the clear distinction of sexes which all the higher 

 Cryptogamous no less than Phaenogamous plants exhibit, is also mani- 

 fested in those of the simplest structure, viz. in plants consisting of 

 single cells, or of rows or clusters of similar and essentially inde- 

 pendent cells. That is, even these afford examples of 



FIG. 635. Magnified view of two conjugatiDg filamente of Zygnema, showing all the stages 

 of the process by which the cells from two filaments form each a corresponding protuberance, 

 these come into contact, the intervening walls are absorbed, and the contents pass from one 

 cell into the other, condense, acquire an investing menabrane, and so form a spore : the stages 

 are represented from above downwards ; a completed spore is seen at the bottom, on the right. 



