606 



SCIENCE. 



[N. S. Vol. XVII. No. 433. 



tivity and water storage capacity, are tlie 

 more important of the edaphic factors. 



The most important of the adaptations 

 to be studied here are those of the root sys- 

 tems, gross as well as minute. 



We recognize three types, with many 

 gradations between them— the tap-root, the 

 heart-root and the tracing root system. It 

 is evident that the last, shallow-rooted, sys- 

 tem is best adapted mechanically to the 

 shallow soils, but since it must supply itself 

 from the surface, its chances of securing 

 sufficient supplies are limited, hence these 

 species are, relatively speaking, not adapted 

 to dry soils or dry atmospheres. On the 

 other hand, the deep-rooting species can 

 secure water from great distances below 

 ground. They would be naturally what the 

 ecologist calls xerophil in their nature. 

 This term is badly chosen, just as the term 

 hydrophil, for the agriculturist, as well as 

 the horticulturist and silviculturist, has 

 amply proved that most plants love neither 

 dry nor wet conditions, although some are 

 more capable of enduring such extreme 

 conditions. 



The trees of the swamps, or many of 

 them, are good examples of this adaptabil- 

 ity, for they are also often found to occupy 

 the driest soils. They would appear 

 xerophil and hydrophil at the same time, 

 but as a matter of fact they love neither 

 and would thrive much better in such con- 

 ditions as the farmer or the nurseryman 

 prepares for his crop ; it is only in the com- 

 petition with other, better-adapted forms, 

 that the unfavorable sites are left to them, 

 to which they are still able to adapt them- 

 selves. 



Some of the deep-rooters have the ca- 

 pacity of modifying their root system and 

 adapting it to shallow soils. Concerning 

 this practically so important phase of 

 ecology we have little or no knowledge as 

 regards our species. 



The climatic range of a species in the 

 natural field gives, of course, a first clue 

 to its climatic adaptation, but we know now 

 very well that mechanical barriers to prog- 

 ress, inefficiency in transportation, and 

 mere competition with other forms are 

 sufficient to exclude species from a wider 

 field. The black locust is a most striking 

 example, having from a very confined 

 natural field become almost ubiquitous. 

 Moreover, within the broader climatic 

 range the distribution of the species is not 

 only determined by edaphic adaptation, but 

 by local variations of climate, such as are 

 brought about by variable topography. 

 Our species so far have remained largely 

 unstudied from this point of view. Among 

 the minor variable features of local climate 

 it is specially the frost phenomena which 

 are of importance, and knowledge as to 

 what species are liable to suffer or capable 

 of withstanding these, and under what 

 conditions, during various periods of their 

 life from the young seedling to the mature 

 tree, would be most desirable. 



The most important of the variable fac- 

 tors of environment in a forest association 

 is the light, and the adaptability to vari- 

 able light conditions of the members which 

 make up the community is of the utmost 

 interest to the silviculturist, and should be 

 to the plant ecologist. 



But, although the physiological relations 

 of light to plant growth have been studied 

 by botanists, the ecologic relations have 

 been hardly recognized. On this field the 

 ecologists owe an apology to the silvicul- 

 turists for having failed to perceive the 

 importance, which the latter have pointed 

 out and appreciated for the last hundred 

 years. 



Almost the whole art of the silviculturist 

 is based on the recognition of photic adap- 

 tations of the different species. Schimper, 

 in his plant geography, fails even to indi- 

 cate the ecologic character of this factor,. 



