POLLENATION : HETEROSPORY. 353 



cases, permanently enclosed within the sporangium, so that if there were 

 motile sperm cells on the outside of the ovary, they could never reach the 

 egg to fertilize it. 



647. But a modification of the microspore, the pollen tube, enables the 

 sperm cell to reach the egg cell. The tube grows through the nucellus, 

 or first through the tissues of the ovary, deriving nutriment therefrom. 



648. But here an important consideration should not escape us. The pol- 

 len grains (microspores) must in nearly all cases first reach the pistil, in 

 order that in the growth of this tube a channel may be formed through which 

 the generative cell can make its way to the egg cell. The pollen passes from 

 the anther locule, then, to the stigma of the ovary. This process is termed 

 pollenation. 



Pollenation. 



649. Self pollenation, or close pollenation. — Perhaps very few of the ad- 

 mirers of the pretty blue violet have ever noticed that there are other flowers 

 than those which appeal to us through the beautiful colors of the petals. 

 How many have observed that the brightly colored flowers of the blue violet 

 rarely "set fruit"? Underneath the soil or debris at the foot of the plant 

 are smaller flowers on shorter, curved stalks, which do not open. When the 

 anthers dehisce, they are lying close upon the stigma of the ovary, and the 

 pollen is deposited directly upon the stigma of the same flower. This 

 method of pollenation is self pollenation, or close pollenation. These small, 

 closed flowers of the violet have been termed " cleistogamoits" because they 

 are pollenated while the flower is closed, and fertilization takes place as a 

 result. 



But self pollenation takes place in the case of some open flowers. In some 

 cases it takes place by chance, and in other cases by such movements of the 

 stamens, or of the flower at the time of the dehiscence of the pollen, that it 

 is quite certainly deposited upon the stigma of the same flower. 



650. Wind pollenation. — The pine is an example of wind pollenated flowers. 

 Since the pollen floats in the air or is carried by the "wind," such flowers 

 are anemophilous . Other anemophilous flowers are found in other conifers, 

 in grasses, sedges, many of the ament -bearing trees, and other dicotyledons. 

 Such plants produce an abundance of pollen and always in the form of 

 "dust," so that the particles readily separate and are borne on the wind. 



651. Pollenation by insects. — A large number of the plants which we have 

 noted as being anemophilous are monoecious or dioecious, i.e. the stamens 

 and pistils are borne in separate flowers. The two kinds of flowers thus formed, 

 the male and the female, are borne either on the same individual (monoe- 

 cious) or on different individuals (dioecious). In such cases cross pollenation, 



