304 



A TEXTBOOK OF BOTANY 



[Ch. VI, 7 



i I 



III. The Rockweeds, the common brown seaweeds so 

 prominent on sea coasts at low tide, and some other Algse of 

 higher grade than those mentioned under II, produce two 

 Ivinds of reproductive cells, one relatively large, round, and 

 without swimming appendages, the other small, elongated, 

 and adapted to swim freely (Fig. 21()). Both kinds when 

 ripe are thrown into the water, where the large cells float 

 passively about while the small cells swim to them and fuse 

 with them, quite in the manner of fertilization in the higher 

 plants ; and this fertilized cell grows into a new plant. 

 ^ We call the larger the egg cell, 



-'':''_ or EGG, and recognize it as 



female, and the smaller the 



SPERM CELL Or SPERMATOZOID, 



and recognize it as male ; and 

 herein we have a clear case of 

 the existence of se.x. Consid- 

 ering, now, the nature of the 

 differences between the two sex 

 cells, it is evident that the egg 

 cell owes its great size to the 

 large supply of food it contains, 

 this food being used in the de- 

 velopment of the new plant un- 

 til it can make its own supply ; 

 and since it is thus large and clumsy, so to speak, its capacity 

 for free locomotion is diminished, and e^'cn the attempt is 

 abandoned. The sperm cell, on the other hand, consists 

 of little more than a nucleus, with onlj' enougli cj'toplasm to 

 construct an efficient swimming apparatus. Here, as in 

 the higher plants, the two nuclei appear to contribute through 

 their chromosomes exactly alike to the offspring, and it seems 

 clear that the difference between the two cells consists in a 

 division of la1)or with respect to t^'o sulisidiary features 

 of reproduction, viz. the bringing of the sex cells together, 

 and the provision of food for the resultant offspring, — one 



A 



Fig. 216. — An egg cell of 

 Rockweed, surrounded by sperm 

 cells, one of which enters and effects 

 fertilization ; X 500. (Redrawn 

 from L. Kny.) 



