230 THE LIFE OF THE PLANT 



In order that fertilisation may take place, the pollen 

 grain must first of all reach the surface of the stigma, 

 to which it easily adheres by reason of the hairs and 

 sticky fluid. The means by which this end is achieved 

 in Nature will be studied later on ; in artificial breeding, 

 in horticulture, it is found convenient to convey the 

 pollen to the stigma with a brush. What happens next ? 

 The distance between the surface of the stigma and the 

 ovule is still great. In what way does the pollen affect 

 the latter ? This question preoccupied botanists for 

 a long time, and gave rise to a series of more or less 

 fruitless suggestions. It was supposed that the pollen 

 grains sink down to the ovary ; that they burst on 

 the stigma and let out their contents which reach the 

 ovule ; even that they act from a distance by means 

 of some kind of emanation. Not one of these hypotheses 

 proved true when exact microscopic investigations at 

 last solved the problem. 



Whenever a pollen grain reaches the stigma or a 

 suitable liquid, say a solution of sugar (but not water, 

 in which it generally bursts), it begins to germinate, 

 the inner wall protrudes through one of the pores in the 

 external wall as a tube (fig. 65, in the centre), into which 

 the contents of the grain are conveyed. The tube goes 

 on growing at the apex and reaches a considerable 

 length. As it grows forward, it often dies behind. These 

 pollen-tubes push their way down through the porous 

 tissue of the style (fig. 65, right-hand side), sometimes for 

 a long distance, as in the cactus, for instance, where the 

 style is several inches in length. Once inside the ovary, 

 the pollen-tubes reach the entrance of the ovule, make 

 their way to the nucellus down the canal and gain 

 entrance to the embryo-sac (fig. 65 right-hand side ; 

 at the bottom of the left-hand side is the upper part of 

 the nucellus, showing the pollen-tube applied to the 

 embryo-sac). The pollen-tubes reach the canal of the 

 ovule almost haphazard ; but the accident happens 



