Ch. VI, 7] 



SIGNIFICANCE OF SEX 



305 



r 



cell assuming wholly the one function, and ^/J 



the other the other. No differences occur 

 in the plants which produce these cells, 

 excepting in the parts immediately con- 

 nected with the formation of cells of such 

 different sizes. Thus we have a stage in 

 which there is a clear distinction of sex, but 

 only in the sexual cells themselves, and it 

 arises not from any fundamental matter of 

 difference in contribution to the constitution 

 of the offspring, but in a secondary matter 

 of division of labor in connection with the 

 mechanism of fertilization, and the nutrition 

 of the resultant embryo. 



IV. The higher, or Red, Alga 

 have a complicated reproduction 

 under which we can recognize the 

 essential fact that the egg cell, 

 naked as in the lower kinds, remains 

 permanently attached to the parent 

 plant, upon which it is fertilized by 

 a much smaller floating sperm cell, 

 and from which the resultant 

 growth is supplied with food (Fig. 

 217). Thus we have a stage, not, 

 it is true, exactly represented in 

 living forms, but presumably once 

 occurring in kinds now extinct, 

 wherein the egg cell remains at- 

 tached to the parent plant, on 

 which it is fertilized and by which 

 the resultant equivalent of an em- 

 bryo is supplied with food. 



V. The stage just described is 

 the highest attained by the Algae. (After l. Kuy.) 



In the simplest land plants, the Bryophytes and Ferns 



Fig. 217. — The egg cell, 



attached to a fragment of 

 frond, of Nnnalion multi- 

 fiduiti, a seaweed ; X 700. 

 Extending from the egg cell 

 is the long-projecting " tri- 

 chogjiie," adapted to receive 

 the small floating sperm cells, 

 of which two are attached. 



