THE MOVEMENTS AND HABITS OF PLANTS. 17 



rope ; and this appears to be the result of the direct action 

 of climate on the reproductive organs. In the best ac- 

 count which I have seen, it is stated that many of the 

 ■varieties in the United States consist of three forms, 

 namely, females, which produce a heavy crop of fruit; 

 of hermaphrodites, which "seldom produce other than 

 a very scanty crop of inferior and imperfect berries " ; and 

 of males, which produce none. The most skillful cul- 

 tivators plant "seven rows of female plants, then one 

 row of hermaphrodites, and so on throughout the field." 

 The males bear large, the hermaphrodites mid-sized, and 

 the females small flowers. The latter plants produce few 

 runners, while the two other forms produce many ; con- 

 sequently, as has been observed both in England and in 

 the United States, the poUeniferous forms increase rapidly 

 and tend to supplant the females. We may therefore 

 infer that much more vital force is expended in the pro- 

 duction of ovules and fruit than in the production of 

 pollen. 



CAUSES OF STEEILITT AMONG PLANTS. 



The Differ- If the sexual elements belonging to the 



Jlovrer""^ ° same form are united, the union is an illegiti- 

 page 346. mate one, and more or less sterile. With di- 

 morphic species two illegitimate unions, and with trimor- 

 phic species twelve are possible. There is reason to be- 

 lieve that the sterility of these unions has not been spe- 

 cially acquired, but follows as an incidental result from the 

 sexual elements of the two or three forms having been 

 adapted to act on one another in a particular manner, 

 so that any other kind of union is ineflBcient, like that 

 between distinct species. Another and still more remark- 

 able incidental result is that the seedlings from an ille- 

 gitimate union are often dwarfed and more or less com- 



