116 PLANT PHYSIOLOGY AND ECOLOGY 



the latter is left only its characteristic task, the production of 

 gametes. During this time a division of labor occurs by which 

 each gamete is developed upon a special gametophore. With 

 the appearance of the flowering plants, the primitive relation 

 in which the sporophore is dependent upon the gametophore 

 is reversed. The macrospore is retained upon the sporophore, 

 and the male gamete is no longer able to reach the egg-cell by 

 swimming through the water to it. The sporangia in which micro- 

 spores and macrospores are produced are further protected and 

 nourishal by being enclosed within the sporophylls that bear 

 them, giving rise respectively to stamens and pistils. A further 

 step in the increase of parental care leads to the loss of the power 

 to produce sporangia by some of the sporophylls, which thereby 

 become sepals. The need of insuring the transfer of pollen grains 

 from stamens to pistils has apparently produced a further division 

 of labor. The innermost sepals have become brightly colored 

 in the majority of cases, and as petals serve as organs of attraction 

 for insects, as well as for other animals that bring about pollina- 

 tion. The flower is hence to be explained as a reproductive 

 device, by which the sporophore secures better protection and 

 nutrition for its spores and gametes, and insures the fusion of 

 the latter in spite of changed conditions and the loss of motility 

 in the male gamete. 



The immediate task of the flower is to bring about pollination 

 and consequent fertilization, by means of which seeds and fruits 

 are produced. To secure the proper discharge of these functions, 

 the flower has undergone innumerable modifications. All of 

 these may be grouped with respect to: (1) the production of 

 pollen, (2) its protection, (3) the disposition of stamens and petals, 

 (4) source and destination of pollen, (5) relation to the agent 

 concerned in dispersal. 



140. Production of pollen. Pollen grains are commonly 

 exposed to the double risk of injury by weather and of loss in 

 transit, particularly in transfer b}^ winds. Furthermore, they 

 often serve as food for the insect agents of pollination, and a large 

 number of grains are thus sacrificed in order that a few may be 

 carried. As a general rule, the amount of pollen produced 

 increases with the danger of loss. There are few if any definite 

 modifications for this purpose, doubtless because it is most 

 easily accomplished by increasing the number of stamens in each 



