304 Yapp. — On Stratification in the Vegetation of a 
the characteristic rounded outlines of desert shrubs 1 ; the cushion and 
rosette plants of alpine regions, &c. All these have long been known. 
Their regular outlines are, of course, due to the growth of the individual 
twigs or leaves to about the same external level. Under extreme con- 
ditions all twigs which project beyond this general shoot-level may be 
killed off. 2 This adds further to the severity of the general contour. 
Now a great deal has been written on the subject of competition 
amongst plants. But, so far as I am aware, with the exception of the cases 
already mentioned (i. e. of plants growing under unusually severe con- 
ditions), comparatively little on the mutual protection afforded by the 
association or massing of shoots at the same level. 
Warming, 3 in contrasting animal and plant associations, says, £ Nur im 
uneigentlichen Sinne kann man sagen, dass gewisse Individuen einander 
beschiitzen \ and he instances the case of wind-swept trees. Later on, 
however, he says further, ‘ Es giebt in den Pflanzenvereinen ganz gewiss 
oft (oder immer) eine gewisse natiirliche Abhangigkeit und eine gegenseitige 
Riicksicht der vielen Glieder eines Vereines von- und aufeinander/ I am 
inclined to think the latter statement deserves more prominence than is 
sometimes assigned to it. Thus, at least in the more extreme cases already 
instanced, it is obvious that the mutual protection derived from the close 
association of shoots at a given level is not only advantageous, but neces- 
sary. But I shall now attempt to show that even in the case of more 
favoured vegetation the difference is merely one of degree. 
It may be said that in general shoots exhibit, in nature, a more or less 
gregarious habit. Thus it is a matter of common observation that her- 
baceous plants, as well as shrubs and trees, tend to assume a more compact 
form when solitary than when growing in close proximity to other plants. 
But what is true of isolated individual plants is true also of vegetation. 
A wood or forest, for instance, generally exhibits a very regular undulating 
superficies : and a field of corn, where the dominant plants are all of one 
species, clearly shows a similar general vegetation level. It has also been 
shown that even in the mixed vegetation dealt with in this paper most of 
the shoots grow to about the same height (see PL I, Fig. i, and Text-figs. 
4 and 6). 
Of course the uniformity of contour is much less in the case of mixed 
associations growing under more favourable conditions than in the extreme 
cases cited above. For instance, the general outline is frequently obscured 
by projecting inflorescences, or vegetative shoots of the (presumably) more 
hardy species. Further, where e. g. shrubs and herbs are intermingled, the 
1 Cf. photos of desert shrubs ; Weiss and Yapp (’06), Pis. V, VI, and VII. 
2 Many figures showing this have been published. It can be readily observed in the case 
of wind-swept Hawthorns, &c., round our own coasts. 
3 Warming (’96), p. no. 
