INFLUENCE OF ENVIRONMENT ON PLANT LIFE. 101 



A combination of the time factor and that of intensity is, frequently 

 if not always, the effective condition which determines the success or 

 failure of plants in nature. With a somewhat rapid alternation of 

 favorable and unfavorable conditions, where the unfavorable factors 

 for any short period do not in themselves at first produce death, the 

 organism may generally lose its power of resistance and finally go to 

 the ground. In regions where such rapid fluctuations in the intensity 

 of external conditions occur, the natural vegetation must necessarily 

 comprise only those forms which can bear this sort of fluctuation. 

 Alpine plants are reputed to be especially resistant to the great daily 

 ranges of temperature that occur in their habitats, and many plants, 

 such as lichens and certain liverworts and club-mosses, exhibit a high 

 power of resistance to repeated wetting and drying-out. 



The limits for life, growth, or reproductive activity (the latter a 

 sort of growth) define the resisting power of an organism in respect to 

 the particular condition considered, and, by maintaining the quality 

 and intensity of the other environmental factors and causing the one 

 in question to vary, the range between the limits for that one may be 

 more or less approximately determined. But, with two or more 

 factors varying at the same time, the problem of physiological limits 

 becomes much more difficult. The study of the behavior of plants 

 when several factors are simultaneously in a state of change has, as 

 we have pointed out, only just begun. We may be sure, however, 

 that the resisting power of a plant to any single condition will prove 

 to be markedly'- influenced by other concomitant conditions. The 

 antagonistic action of certain salts, such as those of calcium and 

 magnesium, in the work of Loew, Osterhout, and others, is a case in 

 point, as is also the well-known fact that, by a degree of desiccation 

 that does not surpass the death-limit, the power of many organisms 

 to withstand both high and low temperatures is markedly increased. 

 The common experiment comparing the effect of high and low temper- 

 atures upon dry and moist seeds is a clear illustration of the latter 

 case; within limits, the less water a seed contains, the more freezing 

 or heating it can bear without losing its vitality . 



III. RELATION OF PLANT DISTRIBUTION TO THE PHYSIOLOGICAL 

 LIMITS OF THE VARIOUS DEVELOPMENTAL PHASES. 



Each plant-form, each developmental phase, and each physiological 

 process exhibits a minimum or a maximum, or both, for each of the 

 environmental factors, these limits depending, of course, on the other 

 conditions that prevail within and without the plant. Whenever an 

 environmental factor falls below the minimum for life, in either quahty, 

 intensity, or duration, the annihilation of the organism of course 

 ensues. A like result follows any increase above the life maximum. 

 It is only with each of the en\ironmental conditions falhng between 



