x 
f 
374 
idea that each topographic type ‘has a life history, an origin, youth, 
middle age, and death, he first conceived of the formation as an organic 
whole, which is susceptible to any change modifying its environment. 
He laid emphasis upon the fact that the normal erosive forces change 
the relation of the ground-water level and so, as base level is approached, 
continually bring it nearer to the surface, by which means water becomes 
more and more available for plants. By all these workers ground water 
is recognized as a controlling if not the primary factor. None of the 
authors cited have made any serious attempt to measure the water content 
of the soil and the other physical factors controlling plant distribution. 
It remained for Clements’ to perfect suitable instruments with which to 
measure the factors and, as a result of his investigations, the concept was 
formed “that vegetation is to be regarded as a complex organism with 
structures and with functions susceptible of exact methods of study.” 
This author also developed a system of nomenclature which, with possible 
changes, bids fair to find acceptance by ecologists. His classification is 
based primarily on the physiological ground-water which he calls the 
chresard, by which form he designates the actual water in the soil which 
is used by the plant. This is measured in the field and because of a limit 
to the size of the root mass and its surrounding earth which can readily 
be handled, only herbaceous or small woody plants can be utilized. How 
the chresard of larger plants, too bulky to be handled, is to be estimated 
the author does not state and unless some method is provided whereby 
the chresard of the larger, woody plants can be measured in situ, or at 
least computed, the true chresard of forest formation can not be obtained. 
It is obvious that the chresard varies from season to season. In tem- 
perate zones it is high during the rainy months of the growing season, 
lower during the dry ones, and least during those of winter. If, as 
Clements has done, the average of all the summer months be taken, and 
a classification based on this alone, then the winter condition will be left 
out of consideration and any classification which does not take into con- 
sideration the lowest chresard can not find a general application. There 
must be a recognition of the tropophytic nature of vegetation and 
of an unfavorable and favorable season. his is the case not only in 
temperate but also in tropical regions. In regions of the Tropics 
where the physical conditions, especially the rainfall, are more or less 
uniform throughout the year, the factors measured during one month 
will give an average for the year, but some places in the Tropics have 
pronounced wet and dry seasons. If the chresard measurements should 
7 Clements, | ec. I have not access to Clements’s Development and Structure of 
Vegetation. Bot. Surv. Nebr. 7 (1904). His volume (in preparation) on the 
vegetation of the mountains of Colorado may throw some light on the discus- 
sion which is given below. I have obtained my information concerning his 
methods from the volume quoted at the beginning of this paper. 
