REPRODUCTIVENESS AND SEED GERMINATION 389 



some of the most interesting devices from an evolutionary point 

 of view. Wild melons are climbing plants which support the 

 small fruits by means of tendrils. When the fruit ripens, the 

 tendril or "curl" behind the fruit dies and breaks releasing the 

 fruit, which falls to the ground with a crash and scatters the seeds. 

 The melons of cultivation are much too large to be supported by 

 tendrils and the plants are therefore allowed to run prostrate on 

 the ground, but the tendril behind the fruit still becomes brittle 

 and dies when the melon ripens! Some of the lower plants, among 

 which are Pilobolus and the ferns already described (Chap. 

 XXIX), have mechanical devices which aid in the dispersal of 

 the reproductive bodies and the spread of the species. 



Seed Germination. — Before seeds will germinate they must 

 have oxygen, water, the proper temperature, and embryos which 

 have reached a suitable degree of maturity. Temperatures that 

 are optimum for growth are generally favorable for seed germina- 

 tion. The warm medium not only favors the chemical reactions 

 that go on within the seed, but also promotes such physical proc- 

 esses as imbibition. But germination may often be hastened by 

 alternating the temperature. Thus Cynodon dactylon, Poa com- 

 pressa, Typha latifolia, and Berberis thunbergii when kept for 6 

 hours at 35-40° and then for 18 hours at 10° C. germinated more 

 rapidly than when kept constantly at the higher temperature 

 (Morinaga, 1924). Coville explains the germination of many 

 plants the following spring by the favoring effect of the low winter 

 temperatures. Cornus canadensis seeds, for example, kept over 

 winter in the greenhouse at a temperature above 55° F. did not 

 germinate when placed in the ground for 12 months, but when 

 chilled for 2 months at 35-40° F. germinated within a month. 

 Similarly Munerati found that 81% of oats germinated right 

 after harvest at 12-15° C. but only 3% germinate at 32-35° C. 



Some seeds, e. g., the castor bean (Ricinus), are equipped with 

 a spongy absorbing organ, the caruncle, which aids in the ab- 

 sorption of water. Parasitic plants, e. g., mistletoe, require for 

 germination that the seeds remain in contact with the correct 

 host which, in some way not completely understood, furnishes the 

 proper stimulus for development. 



Light also plays a part in the germination of many seeds. Ac- 

 cording to Mitchell (1926) about 60% of all seeds tested are favored 

 by light, including the sundew (Drosera), tobacco, mullein (Ver- 



