36 



PLANT LIFE. 



together to form a reproductive cell or spore, illustrates what 

 is meant by the term cross-fertilization, and the present type 

 illustrates a primitive form of sexual 

 reproduction, characterized by the 

 two conjugating cells being almost 

 or completely alike morphologically 

 or in structure ; yet there is evidence 

 for supposing that the two cells are 

 of different functional values, as in 

 the case of Spirogyra, a fresh-water 

 alga common in our ponds and 

 ditches, a plant consisting of a 

 single row of cells placed end to 

 end, and at the period of conjuga- 

 tion two such plants lie parallel to 

 each other ; opposite pairs of cells, 

 belonging to the two individuals 

 respectively, send out on the side 

 nearest each other, a finger-like ex- 

 tension of the cell-wall due to local 

 growth ; these extensions of the two 

 cells eventually meet, the cell-walls 

 become absorbed at the point of 

 contact, thus effecting an open com- 

 munication between the two cells ; 

 through these connecting channels 

 the protoplasm from all the cells of 

 one of the two plants — the male 



Fig. 6. — ( X 400). Conjugation of the cells of two individuals of Spirogyra, 

 a fresh-wEcter alga. A, two cells prepared for conjugation ; at a the 

 filaments have begun to swell 'towards each other ; cl, spiral bands of 

 protoplasm ; K, nucleus. At B the protoplasm of the cell f is fusing 

 with that of the cell *'. Z is a perfectly-formed zygospore which has 

 secreted a cell-wall. (From Prantl.) 



