104 ENVIRONMENTAL CONDITIONS. 



ditions change and the powers of resistance also vary. Thus modified, 

 the sieve analogy becomes unwieldy and does not aid us very much in 

 our thinking. 



Since the behavior of a plant is nothing but the summation of the 

 behaviors of its active parts, and since all higher plants exhibit, at 

 any given phase in their growth, various gradations in the activities 

 of their different tissues, it follows that any adequate consideration 

 of the physiological limits of plant activity as a whole must be ex- 

 ceedingly difficult. It is therefore impossible, in the present state of 

 our knowledge, to treat the question of complex limits quantitatively. 

 The best that may be done in the discussions which follow is to attempt 

 to bring together a series of confessedly incomplete and exceedingly 

 inadequate treatments of the main environmental factors and their 

 general mode of action upon ordinary autotrophic land-plants. 



IV. GENETIC CONTINUITY OF PROTOPLASM AND ITS CYCLIC ACTIVI- 

 TIES. IN CONNECTION WITH PROBLEMS OF DISTRIBUTION. 



In the preceding sections of this chapter the general terms of the 

 problem of plant distribution have been presented in the words of the 

 present-day physiology of plant organs. There can be little doubt 

 that the day of this organic physiology is about to pass. It has been, 

 of necessity, mainly descriptive and has not concerned itself prima- 

 rily with the details of the actual causes of plant response and their 

 mode of action, but there is already a strong tendency to turn attention 

 from the description of plant organs and their responses to the physical 

 causation of these, a development of physiology which bids fair to 

 place this branch of science on the same quantitative etiological basis 

 as that upon which physics and chemistry are now working. From 

 the current literature of plant distribution and of ecology in general 

 it is suggested that many workers in this field have so far failed to 

 realize the present status of the physiology which lies at the base of 

 all ecological facts. Ecology, which was at first regarded as a purely 

 descriptive study, a mere cataloging of relatively superficial descrip- 

 tions of phenomena and a classification of these, was an outgrowth of 

 taxonomy. But it has advanced with more rapidity, perhaps, than 

 any other branch of science, and it has already accumulated enough 

 descriptions so that a beginning at least in the study of cause and effect 

 has been made. Such a study must, by the very nature of its subject- 

 matter, take account of all the contributions so far made by physiology 

 towards an etiology of plant phenomena in their broader aspects. For 

 a long time the physiology of organs must be the basis of ecological 

 considerations, and it is with this in mind that we have taken the 

 principles of organic physiology as the basis of our discussions. We 

 have consciously avoided such ideas as that of purposeful adaptation 

 and other teleological conceptions, still too common in botanical writ- 

 ing — with what success the reader will best be able to judge — and have 



