242 The Ohio Naturalist. [Vol. XIV, No. 4, 



Examples of a more local nature are cited in Bulletin IG, 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, arc detennined 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 themseh'es 

 as dominants or in competition with the plants constituting 

 the association. 



The tenn "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 dr}^ 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 ])roduction or for the probable future population 

 it may support; any study of the geographical mo\'ement of 

 vegetation, if made on the basis of this standard of water use 

 in relation to plant growth, must be influenced by the congruit}' 

 of the rclationshi]) and the magnitude of the value involved. 



