366 MR. T. W. WOODHEAD ON THE 
II.—Errrcr or ENVIRONMENT ON STRUCTURE. 
From what has been said, it is clear that these common species 
must frequently grow under very dissimilar conditions, and in 
examining the plants in the transition zone, or zone of tension 
as Cowles terms it, it is at once obvious that a change of 
conditions, while not immediately limiting their distribution, 
produces a marked effect on their habit and structure. We find 
that as the Mesophytes invade the region of the Xerophytes, and 
come under the influence of drier and more rigorous conditions, 
they develop xerophytie characters. On the other hand, as 
the Xerophytes encroach on the Mesophytes, and come under 
the mellowing influences of moisture and shade, they tend to 
lose xerophytie characters and take on mesophytie characters. 
The more plastie or adaptable a species is, the wider its range 
of variation and distribution ; theless plastie or adaptable species 
show a narrow range of structural variation and a more restricted 
distribution. Between these extremes we find every degree of 
modification. My object has been, therefore, to ascertain the 
region of maximum development of a few common species, study 
their eonditions of growth, and determine to what extent a 
change in one or more of these conditions has upon their 
structure. 
Judging from a summary of the investigations made by 
Chrysler (13), similar changes have been noted in the strand- 
plants occurring on the Atlantic Coast in the vicinity of Woods 
Hole, Mass., and also near Lake Michigan near Chicago, III. ; 
and Hesselman (47) has recently published the results of his 
investigations of the plants of the “ Laubwiesen" of Sweden, 
and they agree closely with mine. 
The study of Plant Biology has of late received considerable 
attention, and the works of Schimper, Goebel, Wiesner, Stahl, 
Haberlandt, &c. provide us with a useful foundation on which 
to build further investigations into the complex relations and 
life-histories of the plants forming these associations. 
There is perhaps a danger, when studying the structure of a 
plant in relation to its environment, to assume on insufficient 
grounds direct adaptation. A useful and timely corrective in 
this direction is the recent excellent criticism by Detto (23). 
The point of view adopted by Küster (54) is also worthy of 
careful eonsideration in this connection. Many of the modi- 
