112 
PLANT STUDIES 
FIG. 110. 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 
hi 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 FlG . m . Apodoffireweed 
blend to form the eggS. Conspicuous (EpUobium) opening and 
among the means of transfer are the exposing its plumed seeds 
. which are transported by 
following. the wind.-After BEAL. 
