EEPKODtrCTIVE ORGANS OP PLANTS. 



295 



volvulus. The cut represents a magnified section length- 

 wise through the short pistil ; a, is the stigma or summit 



are pollen tubes 



of the pistil ; 5, are grains of pollen ; 

 that have penetrated into the seed- 

 vessel which forms the base of the 

 pistil; one has entered the mouth of 

 the rudimentary seed, g, and reached 

 the embryo sack, e, within which it 

 causes the development of a germ ; d, 

 represents t!5e interior wall of the 

 seed-vessel; h, the base of the seed 

 and its attachment to the seed-vessel. 



Darwin has shown that certain 

 plants, which have pistils and stamens 

 in the same flower, are incapable of 

 self-fertilization, and depend upon in- 

 sects to carry pollen to their stigmas. 

 Such are many Orchids. 



Artificial Fecundation has been 

 proposed by Hooibrenk, in Belgium, 

 as a means of increasing the yield of certain crops. Hooi- 

 brenk's plan of agitating the heads of grain at the time 

 when the pollen is ripe, in order to ensure its distribution, 

 which is done by two men traversing the field carrying a 

 rope between them so as to lightly brush over the heads, 

 appears to have been found very useful in some cases, 

 though in many trials no good effects have followed its 

 application. "We must therefore conclude that agitation 

 by the winds and the good offices of insects commonly 

 render artificial assistance in the fecundating process en- 

 tirely superfluous. 



Hybridizing. — As the union of the sexes of diflTerent 

 kinds of animals sometimes results in the birth of a hybriJ, 

 so among plants, the ovules of one kind may be fertilized 

 by the pollen of another, and the seed thus developed, in 

 its growth, produces a hybrid plant. In both the animal 



Fig 63 



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