112 



PLANT RELATIONS. 



Fig. no. A pollen grain (spore) from the 

 pine, which develops wings (w) to assist 

 in its transportation by currents of air. 



It is evident that for 

 the germination of seeds 

 light is not an essential 

 condition^ for they may 

 germinate in the light or 

 in the dark ; but the need 

 of heat, moisture, and 

 oxygen is very apparent. 

 The amount of heat re- 

 quired for germination 

 varies widely with different 

 seeds, some germinating 

 at much lower tempera- 

 tures than others. Every 



kind of seed, or spore, or egg has a special temperature 



range, below which and above which 



it cannot germinate. The two limits 



of the range may be called the 



lowest and highest points, but be- 

 tween the two there is a best point 



of temperature for germination. The 



same general fact is true in reference 



to the moisture supply. 



74. Dispersal of reproductive bodies. 



— Among the most striking external 



relations, however, are those con- 

 nected with the dispersal of spores, 



gametes, and seeds. Spores and 



seeds must be carried away from the 



parent plant, and separated from 



each other, out of the reach of 



rivalry for nutritive material ; and 



gametes must come together and 



blend to form the eggs. Conspicuous 



among the means of transfer are the 

 following. 



Fig. 111. A pod of flreweed 

 {Epihbiuin) opening and 

 exposing its plumed seeds 

 which are transported by 

 the wind. — After Beal. 



