224 



SCIENCE 



[N. S. Vol. XL VI. No. 1184 



eate instruments. The work is pursiied 

 both in the field and in the laboratory, and 

 under experimentally controlled conditions, 

 as well as under natural. The great task of 

 ecology and the purpose of its observation 

 and experimentation lies in the interpreta- 

 tion of the phenomena and the deduction 

 from these data of the general principles 

 underlying the reaction of plants to their 

 environmental factors. 



B. Content of General Ecology. 1. Aut- 

 ecology. — This branch of ecology studies 

 the plant as an individual, and is largely 

 phj'siological in nature. It considers the 

 general results of the relation of the plant 

 to its environmental factors, as shown in 

 the division of plants into great classes ac- 

 cording to their reaction to each of the lead- 

 ing factors. 



These reactions come under three heads: 

 First, the reactions in the activity only of 

 the plant, as the increase of activity under 

 favorable conditions and its diminution and 

 even stoppage under adverse conditions. 

 This group really belongs under the head 

 of physiology, but when considered in the 

 field under natural conditions it may be re- 

 garded as within the scope of ecology. Sec- 

 ond, the effects on plastic tissues or organs 

 of the plant. These may also be produced 

 experimentally and frequently have an im- 

 portant bearing on the economic value of 

 cultures. Third, the effects on permanent 

 structure and function of plant organs. 



"Whatever may be our belief as to the 

 method by which variations are produced 

 and fixed in plants, it is evident that struc- 

 tures correspond more or less to function 

 and are conditioned directly or indirectly 

 by the environment. A comparative study 

 of plants in different habitats leads us to 

 identify or construct from the imagination 

 certain "normal" or original types of or- 

 gans. We find also modifications of these 

 types, which are either temporary, where 



the plant tissues are plastic ; or permanent, 

 constituting variations. In tracing the 

 correspondence of these changes to environ- 

 mental differences we look for and fre- 

 quently think we find what may be called 

 ecological causes. 



Plants are classified according to these 

 modifications, both plastic and permanent, 

 on the basis of the factor which seems to be 

 chiefly responsible for the change. Chief 

 among these is the moisture relation, ex- 

 pressed in the more or less familiar division 

 into hydrophytes or water lovers, xero- 

 phytes or dry-climate plants, and meso- 

 phytes inhabiting an intermediate habitat. 

 A similar relation to light and temperature 

 divides plants into sun-tolerant and shade- 

 tolerant, heat-tolerant and cold-tolerant, 

 groups. The relation to the chemical ele- 

 ments in the soil is not so marked as was 

 once thought to be the case, yet we still hear 

 such words as " calciphiles " and "calci- 

 phobes," and the terms probably represent 

 to a certain extent a real situation. The 

 best illustration of this is shown in a com- 

 parison of organs, especially leaves, of hy- 

 drophytic as compared with xerophytic and 

 mesophytic plants. Here there seems to be 

 a very distinct correspondence between 

 structure and the markedly different en- 

 vironments of these different habitats. 



2. Synecology, which studies plants in 

 the mass, is largely concerned with distri- 

 bution of plants, and may be regarded as 

 an application of autecologj' in the group- 

 ing of plants within greater or smaller 

 areas of the earth's surface. It may be di- 

 vided into (a) " Phy togeography, " in 

 which the groupings are regional and the 

 result of climatic factors, and (&) "Physio- 

 graphic Ecology," in which the groupings 

 are local, as the result of physiography 

 with attendant climatic modifications. 

 These groupings are called plant associa- 

 tions and the fact that different associations 



