MORE ABOUT POLLINATION 305 



there evidently must be a crossing from one plant to an- 

 other if pollination is accomplished at all. 



(3) The pollen may cross to the stigmas of flowers of a 

 different kind of plant. Usually in such a case a pollen 

 tube is not produced, but sometimes it is. Plants of just 

 the same kind form what are called species. (The singular 

 and plural of this word have the same form.) When the 

 pollen of one species germinates upon the stigma of another, 

 it is more than ordinary cross-pollination ; it is the crossing 

 of one species with another. Only nearly related species 

 cross in this manner; crossing occurs frequently, for ex- 

 ample, among the different species of oak. Plants pro- 

 duced from seeds which have resulted from the crossing 

 of species are called hybrids. Hybrids are sometimes un- 

 able to produce fertile seeds ; crossing appears to weaken 

 their reproductive power. The production of hybrids has 

 been of much importance in the cultivation of plants, es- 

 pecially in floriculture. 



C. The Stigma. Stigmas are not definite organs so 

 much as they are " regions for the reception of pollen." 

 They show much variety in form. Think of the small 

 knob-like stigmas of many common flowers as compared 

 with the silk of corn or with the feathery stigmas of some 

 grasses. (See Figures I2j and 128.) The stigmas of wind- 

 pollinated flowers are as a rule larger and more promi- 

 nently exposed than those of insect-pollinated ones. Sticki- 

 ness, on the other hand, is more characteristic of stigmas 

 which are pollinated by insects than it is of those which 

 are pollinated by the wind. 



When flowers are abundant there is much pollen in the 

 air. It is the flying pollen of some kinds of plants which 



