DEVELOPMENT OF PLANTS 



181 



smallness promotes their movements through the water. Note 

 also that these results are obtained without any additional 

 expenditure of material the 200 male gametes being produced 

 from a cell no larger than Sphaerella w r here only a few are found. 

 The increase in the size of the female gamete renders her move- 

 ments more difficult, but the storage of food in this gamete pro- 

 vides for the better nourishment of the next generation. Finally 

 the nourishing function becomes so strongly developed that the 

 female ceases to move at all and remains protected in the mother 



B 



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 col- 

 onies 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 neigh- 

 boring 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 gametes. F, male gametes 

 enlarged. 



cell. Thus we see that sexuality may have arisen among plants 

 as the result of definite stimuli as heat, moisture, light, food and 

 other conditions that occur in the environment. These stimuli 

 produce enfeebled zoospores that are incapable ordinarily of 

 growth unless a fusion of two of them is effected. So sexuality 



