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1907] BRIEFER ARTICLES 343 
of that plant extends itself deep into the ground; the structure is a visible 
physiological result, and is to be measured and expressed like the other 
physiological processes. 
4. We must especially learn to distinguish and to express to what degree 
the various physiological processes are plastic to the action of stimuli, and 
to what extent are limited by heredity. 
5. We must classify the physiological processes for study. Ecologi- 
cally speaking there are four critical periods in the life-cycle of a plant. 
These are (a) the germination of seed or spore, (0) the orientation of the 
seedling whereby the plant gains a grip upon its immediate surroundings, 
(c) the expansion of the adult, and (d) flowering and fruiting, or sporifica- 
tion. It is not enough that the plant can match an environment in three 
of these; it must match in all four. I have no doubt dissemination brings 
regularly into a certain habitat some plants which can germinate their 
seeds there, flourish as adults, flower and fruit, but in which the seedling 
stem and leaves cannot stand the light and heat to which they are exposed, 
or in which the seedling root cannot reach the permanent water supply, and 
accordingly that kind of plant cannot exist in that habitat. I think it is 
some such determinant which brings it to pass that, of all the trees whose 
seeds are brought upon the Miscou sandplains from the neighboring 
upland, not one can exist there except the white spruce. I think many 
peculiarities of ecological distribution are determined by just such single 
seemingly insignificant causes. But to explain such cases we must know 
the plants, and must know them throughout their life-cycles; and this is 
the essence of a knowledge of physiological life-histories. 
The particulars of a classification of physiological-ecological processes 
can be worked out only by experience; but following are some of the more 
important topics: 
A. Germination of the seed: 
Amount of water requisite, least and best, and duration thereof. 
Temperature requisite, minimum, optimum, and maximum, and rate under 
Light, whether necessary, injurious, or indifferent. 
Quality of water requisite; how much of salts it may contain. 
Free oxygen requisite, or can it germinate under water ? 
Germinates on the surface or needs planting, and why? 
The seedling life: 
Root system; ordinary form and size and limits of irritable adjustment. 
Stem and leaf system, ordinary form and size, and limits of irritable 
adjustment. 
Temperature conditions. 
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