438 PLANT PHYSIOLOGY 



inner coat (intine) is stretched into a long tube that descends 

 through the style, thus reaching the ovule. The pollen of the 

 majority of plants can germinate not only on the stigma but also 

 in artificial media, especially in sugar solutions at a concen- 

 tration of 2 to 30 per cent. The pollen of a few plants can 

 germinate in water. In the majority of plants, it will rupture 

 and die when placed in water alone. Probably this is owing to 

 the very high osmotic pressure that develops inside the pollen 

 grain. 



The substances necessary for the development of the pollen 

 tube, nitrogenous as well as nonnitrogenous, are previously 

 deposited in the pollen grain, and the length of the pollen tube is 

 evidently determined primarily by the amount of these reserves. 

 A possible supply of nutritive substances to the pollen tube from 

 the tissue of the style is not excluded. Besides, the growth 

 of the pollen tube requires the presence of some specific substance 

 of the type of hormones or vitamins, which are supplied by the 

 style. The pollen tubes of many plants will grow in the tissues 

 of the style much better than in an artificial medium, in which 

 they will make but feeble growth. In many instances, the 

 pollen will germinate only on the stigmas of the same species; 

 and when it does germinate on those of another species, the 

 growth of the pollen tube will be so slow that it never reaches the 

 ovule. The pollen of many plants that require cross-fertiliza- 

 tion, as for instance fruit trees, produces very poor growth on 

 the stigmas of the same individual, while it may grow excellently 

 on other varieties of the same species. It is supposed that certain 

 inhibiting substances are formed by the plant that delay the 

 growth of its own pollen, thereby causing a physiological self- 

 sterility in certain plants. It should be emphasized that this 

 specific effect holds good for the pollen of all plants of the same 

 clone, i.e., individuals obtained by means of vegetative propaga- 

 tion from a single original individual. This fact is of great 

 importance in horticulture and sometimes leads to low yields of 

 fruit in an entire orchard. 



The temperature conditions under which pollen germinates 

 are also of considerable significance for successful pollination. 

 There is a definite optimum temperature for the growth of pollen 

 tubes, and the minimum is often quite high. Cold weather 

 during blossoming time, therefore, has frequently an unfavorable 



