354 TEXTBOOK OF PLANT PHYSIOLOGY 



stigma but also in artificial media, especially in sugar solutions at a 

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

 germinate in water, but in the majority of cases it will rupture and 

 die when placed in this liquid. Most likely this is due to the very 

 high osmotic pressure which exists inside the pollen grain. 



The substances necessary for the development of the pollen 

 tube, nitrogenous, as well as non-nitrogenous, 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 

 without, i.e., from the tissue of the style, is, however, not excluded. 

 Besides, the growth of the pollen tube requires the presence of some 

 specific substances 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-fertilization produces very poor growth on the 

 flowers of the same individual, while it may grow excellently on 

 other specimens of the same species. It is supposed that certain 

 inhibiting substances are formed by the plant which delay the 

 growth of its own pollen, thus causing a physiological self-sterility 

 in certain plants. It should be emphasized that this specific effect 

 holds good for the pollen of all plants obtained by means of vegeta- 

 tive propagation 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 as regards 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 effect 

 on fruit setting. The harmful effect of heavy rains and fogs is due 

 to the fact that pollen subjected to protracted wetting with water 

 will burst before germination. Besides, sugar and other sub- 

 stances necessary for the germination of pollen may be washed off 

 the stigma. 



Since pollen is a resting spore, it will preserve for a certain period 



