Marsh , and its Relations to Evaporation, &c. 305 
taller plants (which, however, generally form their own independent shoot- 
levels) tend to distract attention from the uniformity of level of the shorter 
vegetation. 1 But even in the less obvious cases, a general tendency to the 
formation of ‘ twig or shoot associations ’ may usually be traced. 
With the causes of this phenomenon 2 we are not concerned here. But 
its effect on the transpiring organs have been seen throughout this paper ; 
at least in the case of two slightly differing types of marsh vegetation. 
This effect is to very markedly lessen the atmospheric promotion of tran- 
spiration, even in the upper strata of the vegetation. Further, it is impor- 
tant to note that the majority of the transpiring organs (especially the 
larger ones) are placed below the general vegetation level. It also limits 
the influence of wind. Text-fig. 4 attempts to show how the direct action 
of wind is practically limited to the highest strata of the vegetation. Even 
when strong winds are blowing, the plants of the lower layers are scarcely 
disturbed at all. 
The fact 3 that the exposed tips of the leaves of the sedges and grasses 
die early affords additional evidence of the relative severity of the atmo- 
spheric conditions at or above the upper level of the vegetation. In one of 
his papers Hansen 4 describes an experiment in which he trained vines 
against open espaliers in a garden. The winds were only light, but usually 
just sufficient to keep the leaves slightly moving. By the end of July, 
however, many of the leaves had a complete edge of withered brown tissue. 
On the other hand, the wind had practically no injurious effect on vines 
growing against a wall. 
To sum up, there seems to be a general tendency in nature for vegeta- 
tion, as well as isolated individual plants, to form more or less definite 
‘ general shoot-levels ’. This close proximity of the shoots to each other 
results in a very real (and at least in many cases necessary) mutual protec- 
tion, both against excessive transpiration, and also against the mechanical 
effects of wind. Further, the contours to which such shoot associations 
give rise depend to a considerable extent for their degree of regularity on 
the relative severity of the environmental conditions. 
Thus the structural peculiarities of vegetation, no less than morpho- 
logical or anatomical modifications of leaves and other transpiring organs, 
may be effectual in securing immunity from the dangers of excessive 
transpiration. 
In a later paper it is intended to deal with the anatomical structure of 
the transpiring organs, in relation to the stratification of the vegetation. 
1 e. g., the bushes on a marsh; cf. Yapp (’08), p. 65, also PI. IV, Fig. 1. Gorse bushes on 
a mountain pasture are another example. 
2 e. g., the relative parts played by light, humidity, & c., in determining these growth relations 
of shoots. 
3 Mentioned in Yapp, 1. c., p. 65. 
4 Hansen (’04), pp. 33-4. 
