INFLUENCE OF ENVIRONMENT ON PLANT LIFE. 105 



aimed to place our treatment of plant distribution on a basis as free 

 from anthropomorphic conceptions as is that of physiography, a 

 science with which plant geography must always be closely related. 



But the modern trend of physiology, with its apphcation of the 

 methods and findings of physico-chemistry and its tendency to seek 

 explanations of all phenomena in the properties of matter and energy, 

 offers to the study of distribution at least one conception which goes 

 far to simplify the logic of physiological limits and their modes of 

 variation. The present section will deal with this conception. 



From the standpoint of general physiology, the reproductive activ- 

 ities are to be regarded as a special form of growth. ^ The protoplasm 

 of any species is a continuously existing thing, to be 'likened, perhaps, 

 to a river, in which the material particles are ever changing and of 

 which the form, course, activities, etc., show a continuous variation. 

 The channel of our river moves from place to place within the limits 

 of its valley; the river accomplishes much work of excavation and the 

 like at certain seasons of the year; at other seasons it is inactive and, 

 in the arid regions, often disappears from sight completely, flowing 

 only underground. In the latter condition we may perhaps speak of 

 it as dormant. Following this analogy, the living substance of any 

 species may be thought of as continually existent, but varying widely 

 in situation, amount, activity, etc., with the ever-fluctuating physical 

 conditions within and without its mass. Thus the conditions for 

 the success or failure of any species in any region or habitat are that 

 its protoplasm be indefinitely maintained in that area, and that the 

 various cyclic phases of physiological activity follow one another in a 

 certain order. By such a conception we are enabled to generalize 

 without the complication incident to the special consideration of the 

 reproductive activities. 



Scrutinized in this manner, the individuals of a given plant-form 

 are seen in somewhat the same light as are the buds of an indefinitely 

 growing perennial, such as a tree. These buds are continuously dying 

 and being formed, but the system of growing-points which make up 

 the tree possesses form, size, etc., and maintains itself throughout 

 years. Growth here consists partly in the approximate replacement 

 of parts which have been destroyed by the action of adverse condi- 

 tions, and partly in an actual increase in the number of growing-points 

 comprised within the entire system. 



By this sort of generalization we may bring all plant-forms into the 

 same broad category, as far as the general influence of external con- 

 ditions is concerned. The protoplasm of any species, in any region, 

 passes through variously active and dormant phases with the march 

 of the seasons, the success of a species requiring that there be, at least, 

 no progressive and permanent decrease in the number of growing- 

 points or individual groups of active cells. But there may occur, and 

 usually do occur — outside the tropics at any rate — great periodic 

 fluctuations in the number and activity of these biotic units. 



