REPRODUCTIVE ORGANS OF PIANTS. 



319 



tube which penetrates the interior of the pistil until it 

 enters the seed-vessel and comes in contact with the ovule 

 or rudimentary seed. This contact being established, 

 the ovule is fertilized and begins to grow. Thencefor- 

 ward the corolla and stamens usually wither, while the 

 base of the pistil and the included ovules rapidly increase 

 in size until the seeds are ripe, when the seed-vessel falls 

 to the ground or else opens and releases its contents. 



Fig. 62 exhibits the process of fertilization as observed 

 in a plant allied to buckwheat, viz. , the Polygonum con- 

 volvulus. The cut represents a magnified section length- 

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

 of the pistil ; h are grains of pollen ; 

 c are pollen tubes 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 the interior wall 

 of the seed-vessel ; h, the base of the 

 seed and its attachment to the seed- i 

 vessel. 



Self-Fertilization occurs when 

 ovules are impregnated by pollen 

 from the same flower. In many plants 

 self-fertilization is favored by the posi- 

 tion of the organs concerned. In the 

 pendent flower of the Fuchsia as well 

 as in the upright one of the strawberry the stigma is just 

 below and surrounded by the anthers, so that when the 

 mature pollen is discharged it cannot fail to fall upon the 

 stigma. Some flowers, as those of the closed gentian 

 {Gentiana Andrewsii) and the small subterranean blos- 

 soms of sheep-sorrel (Oxalis acetosella), touch-me-not 

 (Impatiens), and of many violets, never open, and not 



