242 
The Ohio Naturalist. 
[Vol. XIV, No. 4, 
Examples of a more local nature are cited in Bulletin 16, in the 
chapters dealing with the historical factors of bog vegetation 
and the succession of vegetation upon peat soils. It is there 
shown that areal movements of vegetation during remote geo¬ 
logical periods of time as well as to-day, are determined partly 
by the external conditions to which a plant or the social aggregate 
is exposed and partly by the functional limits of the organic 
units, these two sets of factors themselves progressively changing 
as vegetation types evolve. A further consideration of importance 
is the theory entertained that the change of conditions, in the 
remote past, following the accumulation of organic soil (peat-like 
in nature) and the invasion of it by organisms originally aquatic, 
had played a prominent part in the establishment of a land 
flora and the further differentiation of it into those alternating 
phases of the life cycle which are so characteristic of archegoniate 
plants. ' » 
In the work of an experimental nature, the writer brought 
out the fact that the point of most importance which should be 
noted in this connection is the difference in the water require¬ 
ment of plants. The experiments cited showed clearly that 
transpiration is not a measure of growth even under the same 
atmospheric conditions, and can not be looked upon as the most 
striking criterion for such colonists among plants as are steadily 
coming into a new habitat and succeed to establish themselves 
as dominants or in competition with the plants constituting 
the association. 
The term “water requirement” is a word which enjoys the 
advantage of brevity as well as euphony, but it is also another 
instance of the rather numerous cases in the literature of applied 
botany of the misleading use of terms. It is assumed by many 
writers that a definite and quantitative relation exists between 
transpiration and growth, and that hence the ratio of the weight 
of water absorbed and transpired by a plant during its growth 
to the green or dry substance produced is an adequate and simple 
measure of growth. The generalization from the data pre¬ 
sented by them is too broad; it is seriously inadequate to account 
for numerous exceptions in the investigations bearing on this 
subject, and is certain to lead to error. It is needless to say 
that any measure of “the agricultural duty of water, ” of the 
water economy of crops or of native vegetation types; any action 
looking toward the better utilization and management of water 
resources for irrigation; any estimation of the capacity of a 
land area for crop production or for the probable future population 
it may support; any study of the geographical movement of 
vegetation, if made on the basis of this standard of water use 
in relation to plant growth, must be influenced by the congruity 
of the relationship and the magnitude of the value involved. 
