September 7, 1917] 



SCIENCE 



225 



follow each other successively is expressed 

 in the term "Plant Succession." 



C. Special Ecology of Structural Groups. 

 — ^While all ecological groups have more or 

 less specific reactions which are considered 

 under their appropriate heads, there is one 

 grouping which demands separate treat- 

 ment because it is based on the most strik- 

 ing structural feature — the presence or ab- 

 sence of woody tissue, and also because of 

 its practical relation to man's activities. 

 Although verging more closely on agricul- 

 ture, it may still be classed as ecology be- 

 cause the point of approach is from the 

 side of the environmental relations. On the 

 basis of woody structure we classify plants 

 as trees and herbaceous plants with shrubs 

 and lianas occupying an intermediate posi- 

 tion, and it is at once evident that these 

 two groups have decidedly different ecolog- 

 ical reactions. 



1. Ecology of Trees and Shruhs. — This 

 study would involve (a) description of 

 leading species with their habits of growth, 

 characteristic structures, and ecological in- 

 terpretation of the same. This would be 

 the autecology of the group, (h) The syne- 

 eology would involve the distribution and 

 range of the leading species and their rela- 

 tion to ecological causes, (c) We might 

 notice also the influence of the species on 

 their environment as illustrated in the in- 

 fluence of forests on soil moisture content 

 through their control of run-off; and the 

 influence of individual trees, as for ex- 

 ample, the euealj^tus in the reduction of 

 soil water; also the influence of forests on 

 soil in the formation of humus and the 

 effect of trees on wind, as in protection by 

 windbreaks, (d) It could include also a 

 classification of trees according to the char- 

 acter of their wood, including distribution 

 of the different woods and methods of uti- 

 lizing. Also a similar classification accord- 

 ing to the character of their fruits, their 



chemical products and their value for orna- 

 ment. 



2. Ecology of Herbs. — Here should be 

 studied (a) the general characteristics of 

 herbs as distinguishing them from trees, 

 with the ecological differences involved, 

 under the heads of shoot, root, flower and 

 fruit, with the characteristic differences be- 

 tween perennials and annuals; (&) a study 

 of herbs as classified according to their 

 value to man, as : valueless or "wild," those 

 of economic value or "cultivated," and 

 those undesirable or injurious, which we 

 call "weeds." Uncultivated herbs are of 

 interest chiefly synecologically as the asso- 

 ciates of trees in their different groupings 

 and as indicators of the characteristics of 

 the environment, as hydrophytic, xero- 

 phytic, etc. As the subject of taxonomy has 

 to do chiefly with the wild herbs it is fre- 

 quently included under ecology to-day. 



Cultivated herbs and their attendant, 

 though undesirable forms, are considered 

 more from the autecological side. Their 

 reactions to and tolerance of extremes of 

 temperature and moisture and chemical 

 conditions, are of course of chief impor- 

 tance. Original habitat and distributioH 

 and to some extent taxonomic relations, are 

 also important as indicating suitability for 

 certain environments. This value is testi- 

 fied to by the systematic search for new 

 varieties carried on by the United States 

 Department of Agriculture. Herbs vary 

 greatly in their reactions to environmental 

 factors, and should be grouped as far as 

 possible along the lines of similar behavior. 

 EInowledge of these groups should be as 

 complete as possible, but a thorough study 

 of the ecological reactions of a few tj'pe 

 genera and species should be included in 

 any comprehensive course in ecology. 



3. The ecology of lower types of plants 

 is not treated separately, but on account of 



