DEVELOPMENT OF PLANTS 



161 



in the mother cell while other cells form numerous small motile 

 male gametes which are yellowish in color. In Volvox, over 

 200 male gametes may be formed from a single cell (Fig. 103, 

 D,E). 



This tendency to form gametes of different sizes is manifestly 

 of great advantage. The great number of male cells increases 

 the chances of fusion of the gametes and their smallness pro- 

 motes their movements through the water. The increase in the 

 size of the female gamete renders her movements more diffi- 

 cult, but the storage of food in this gamete provides for the 



Fig. 103. Features in the life history of Volvox: A, a colony which 

 may contain as many as 20,000 plants. The three central groups are young 

 colonies which may arise by the repeated division of any of the plants. 

 B, side view of one of the plants showing the canal-like connections with 

 the neighboring plants. C, surface view of a plant with canals radiating 

 out to the adjoining plants. D, a plant enlarging and forming a single 

 motionless female gamete. E, a plant forming numerous small male gam- 

 etes. F, male gamete enlarged. 



better nourishment of the next generation. Finally the nourish- 

 ing function becomes so strongly developed that the female 

 ceases to move at all and remains protected in the mother cell. 

 Thus we see that sexuality arose among plants as the result of 

 12 



