162 



HORTICULTURE FOR SCHOOLS 



fertilized by pollen from some other variety are said to be 

 cross-fertilized. When a flower will not set seed with pollen 

 from itself or from flowers of the same variety, it is self-sterile. 

 When flowers of a particular plant will not set seed with 

 pollen obtained from some other kind, the varieties are said 

 to be inter-sterile. 



In many cases, cross-pollination with the consequent cross- 

 fertilization results in the best seed, as well as in a heavier 

 set of seed and fruits. In many cases also, self-pollination 

 will not result in fertilization at all, so that it is necessary 

 that many plants have flowers with arrangements of some 

 kind to prevent self-fertilization, and to 

 encourage cross-pollination by insects. 

 These devices are exceedingly numerous, 

 varied in structure, and interesting, but 

 cannot be described here. 



260. Requisites for fertilization. In 

 order that fertilization may take place, 

 both the pollen and the pistil must meet 

 certain requirements. The pollen-grains 

 (Figs. 103, 104) must be viable, that is, 

 they must be capable of germinating, of 

 producing a tube, and a cell capable of 

 Fl grai 1 ns. 3 'A7.K?effer uniting with the egg-cell of the ovary. 

 moni; B c?^idp^p^ Many factors affect the viability of pollen : 



D,acabia;E,hazefnut. (1) p ollen mugt be matu re. It must have 



reached a certain age and passed through a definite period of 

 development before it can germinate and fertilize the ovule. 

 This development takes place within the anther, which, as we 

 have seen, cracks open at the proper time and liberates the 

 pollen-grains. On the other hand, pollen-grains which are too 

 old will not germinate, or if they do may not fertilize the ovule. 

 (2) The vigor of the tree affects the viability of the pollen. 

 Healthy trees generally produce better pollen year after year 

 than do sickly, small, stunted ones. (3) The age of trees has 



