i5o8 A TEXTBOOK OF THEORETICAL BOTANY 



valuable genotypes. It would, in fact, have effects somewhat similar to 

 those of autogamy in tending to stereotype the race. 



Lastly there is the very important consideration that a dense local 

 population of a single species is much more vulnerable to epidemic attacks 

 of insects or parasitic fungi. Long-lived seeds are often heavy and devoid 

 of any means of dispersal. Instead of being distributed in space the off- 

 spring are distributed in time. 



The means whereby the dispersal of seeds is favoured are closely 

 similar, in general principles, to those which apply in the case of fruits, 

 and the same agencies, mechanical projection, wind, water and animals, 

 are employed. 



Mechanical Dispersal. First in consideration come the methods of dis- 

 persal by mechanical means. Here the seeds play a passive part and the 

 fruits provide the means of projection. Among dry fruits there are two 

 principal means; either by the release of a spring or by compression. 



In the first category come the legume fruits of many Papilionaceae, in 

 w^hich the two valves, in drying, set up a considerable strain, due to the 

 shrinkage of two superposed layers of crossed fibres, which is violently 



Fig. 1378. — Geranium pratense. Mature fruit 

 in which all five carpels have sprung up- 

 wards. The central column is formed of 

 the inner portions of the five styles and 

 the stigmas may be seen above. 



released w'hen the valves eventually separate. Each valve twists spirally 

 and throws out the attached seeds with some violence. A similar method 

 is found in species of Cardamine. The fruit is a siliqua and the two halves, 



