94 PRINCIPLES OF PLANT CULTURE 



152. Advantages of cross-pollination. — Cross-pollin- 

 ation is advantageous in plants, as Darwin's careful 

 experiments have shown. The seeds formed are usually 

 more numerous and larger and make more vigorous plants 

 than with close-pollination. Especially is this true when 

 the parent plants have been subjected to different growth 

 conditions in previous generations. Nature favors cross- 

 pollination in perfect-flowered plants by numerous adap- 

 tations tending to prevent self-pollination, as maturing 

 the pollen either before or after the receptive stage of 

 the stigma, or so locating the stamens that the pollen is 

 not readily deposited on the stigma of the same flower.^ 

 In some cases, pollen is infertile on the stigma of the same 

 flower or plant that is abundantly fertile on stigmas of 

 other plants of the same species (154). 



153. Perfect, monoecious and dioecious flowers. — 

 Flowers containing both stamens and pistils (or pistil), 

 as in the apple, tomato, cabbage, etc., are called perfect 

 or hermaphrodite; those containing but one of these 

 organs, as in the melon, Indian corn and so on, are called 

 imperfect or unisexual. Flowers of the latter class are 

 called monoecious when the stamen-bearing (staminate) 

 and pistil-bearing (pistillate) flowers are both produced 

 on the same plant, and dioecious when produced on 

 different plants only, as in the hop and date. In a few 

 plants, as the strawberry (154) and asparagus, some in- 

 dividuals produce perfect, and some imperfect, flowers. 



154. Planting with reference to pollination is impor- 

 tant in certain plants. All dioecious plants (153) in- 

 tended for seed or fruit must have staminate and pis- 

 tillate plants growing near together or they will not be 



' Darwin's work "On the Fertilization of Orchids by Insects" 

 describes many most interesting adaptations of this sort. 



