174 



REPRODUCTION 



the prothallus is unisexual and free living, the male anthero- 

 zoid having fertilized an egg, the male gametophyte becomes 

 useless and dies; but the female prothallus has not only to 

 produce the egg, but to nourish it after it has been fertilized. 

 For a time in certain ferns and many fossil plants the mega- 

 spore was dependent on its own resources. Safety was found 

 in the higher seed-bearing plants 

 b}^ retaining the megaspore with 

 its resultant gametophyte (pro- 

 thallus) -svithin the body of the 

 sporophyte. 



The nucellus of the plant 

 corresponds with the mega- 

 sporangium. Within it is the 

 embryo-sac, which grows by 

 feeding on the surrounding cells, 

 and absorbing their contents. 

 Within the embryo-sac are cells, 

 one probably representing the 

 ovum or egg, and the others 

 relics of the primitive prothallus. 

 In flowering plants therefore the 

 prothallus is hidden away inside 

 the tissues of the parent, just as 

 in a mammal the embryo is hid- 

 den away inside the body of the 

 mother. The pollen-sacs of a 

 flower, borne at the free end of 



the stamens, represent the mi- 



rrii • • J. Fig, 63. Pollen-grains (p) germin- 



crosporangia. These give rise to ^^^^ ^^ ^^^ J^^^ of a Grass. 



pollen-grains, which represent After Kerner. 



the microspores. Pollen-grains 



are blown or carried in some way to the stigma of the female 



organs and emit a long pollen tube. The pollen-grain contains 



two cells. One is usually larger than the other. It is called 



the vegetative-cell, and probably represents the vegetative 



part of the prothallus. The other and smaller cell divides into 



two male gametes, each of which represents an antherozoid of 



the original prothallus. One marked advance was gained when 



