282 Yapp . — On Stratification in the Vegetation of a 
(e) Projecting above the general vegetation level may be seen stray 
shoots of Phragmites communis (N.B. This species forms its own general 
level when social), and, if the vegetation is dwarfed, parts of the leafy shoots 
of other species such as Ly thrum, Lysimachia , Spiraea , &c. Many inflore- 
scences, accompanied only by the smallest cauline leaves (e. g. Angelica , 
Spiraea , Calamagrostis , &c.), also project above the general level. 
From the foregoing it follows that not only are there fairly definite 
strata in the vegetation, but also that many of the plants themselves may 
be said, in a sense, to be more or less stratified. In the case of erect 
shoots, as pointed out above, great differences of leaf-development are seen 
at different levels on the same shoot. But the leaves formed at these 
various levels often differ in character as well as in size. This point will be 
dealt with more in detail in a future paper, but the case of species with 
hairy leaves may be mentioned here. In Spiraea Uhnaria , for example, 
the lowest leaves (formed in the springtime), are glabrous, while those 
formed at higher levels are more and more densely hairy. Lysimachia 
vulgaris (Text-fig. 1), Mentha aquatica , and other hairy species, show similar 
relations as regards the position and time of development of glabrous and 
hairy leaves. 1 Thus, as regards the vertical distribution of the transpiring 
organs, even in a plant association where the majority of the individuals 
attain to more or less the same height, differences in habit of the individual 
plants give rise to a marked stratification of the vegetation. It is, of course, 
a commonplace that the organs of the plant, stems, petioles, &c., can 
readily adjust themselves by growth, so that the leaves may be placed 
in each case in the appropriate ecological layer of the vegetation (cf. 
Text-fig. 2). 
Evaporation as a Measure of the Atmospheric promotion 
of Transpiration. 
It is obvious that the physical conditions, i. e. light, humidity, &c., 
must vary to a greater or less extent, in the different strata of the vegeta- 
tion. Further, it would seem probable that such differences, if sufficiently 
great, might have an important bearing on the question of xerophytism in 
marsh plants. So far as I know, no quantitative experiments had been 
previously conducted, such as would afford any definite idea of the actual 
physical conditions obtaining at different levels in a vegetation like that 
dealt with in this paper. It was therefore decided to investigate these 
conditions, in so far as the factors directly affecting transpiration are 
concerned. 
Apart from the question of control by the plant, the physical process 
of transpiration from the external part of the wall of a transpiring cell is 
1 Yapp (’ 07 ), p. 691. 
