BOTANICAL EVOLUTION. 417 
the necessity of forming so much mere supporting tissue, it will 
probably be advantageous to it to give up the formation of this un- 
necessary material Now an immense number of plants have 
developed in this direction, but in trying to save labour in one line, 
they are often obliged to bestow it in another. Perhaps the simplest 
form of this development is that found in chickweeds and similar 
trailing plants. The chickweed is by no means the feeble plant we 
are sometimes inclined to think; it appears to have a considerable 
faculty of smothering its neighbours, and this is apparently one of the 
faculties on which it depends for its success in life. The very limp- 
ness of its stems appears to be an advantage to it, for it is a much 
more aggressive plant than many of its erect-stemmed allies. But 
some plants have even given up the attempt to fight the battle above 
ground, and trust to their engineering powers for their success. 
Some of the worst weeds known have developed very high powers in 
this direction. The creeping thistle, couch-grass, and sorrel send 
their underground stems in all directions through the soil, and this 
faculty is one which has not been evolved in a generation. But the 
majority of the plants which have given up the erect power of growth 
have apparently only done so, when they had begun to use other 
plants for support, and we term all such plants climbers. The study 
of climbing plants itself constitutes a whole chapter in the history of 
evolution. The variety of contrivances adopted is immense, and the 
degree of perfection is very various. Almost every gradation can be 
traced, and I shall endeavour to show you by typical plants how this 
is. The simplest and most rudimentary type of climber is repre- 
sented by this common plant, Muhlenbeckia adpressa, which has not 
developed any special mode of climbing, but appears to straggle over 
the nearest low growing shrubs. All the species in New Zcaland 
are shrubby, but the others are plants which grow prostrate on the 
ground or straggle over low rocks, while this one alone seems to affect 
the edge of the bush and to exhibit the beginnings of a true climbing 
process. In the lawyer (Rubus australis) a considerable differentiation 
has taken place. All the allies of this plant (such as the true roses, 
brambles, rasps, &c.), exhibit a strong development of epidermal 
‘structures in the form of hooks, spines, or hairs, but in none are these 
structures so perfect as in our lawyer. In it the leaf is three or 
four times divided (in one variety reduced to midribs only), and 
the hooked spines are arranged along the under surface and all 
curved backwards. To facilitate their catching powers, the midribs 
as well as the petioles are bent in an upward direction. It is mani- 
fest that if by the shaking of the plant by wind, its hold of the 
support on which it is climbing is loosened, the hooks will only slip 
higher up and hold on again. By this means the lawyer always 
tends to surmount the plant which supports it. 
In the genus Clematis we have a different development leading to 
much the same result. The petioles or leaf-stalks are irritable either 
to prolonged pressure or to gentle friction, and bend in the direction 
from which the pressure or friction comes. When for example, the 
young leaf moved gently by the wind comes in contact with a branch 
of a tree, it takes a turn or two round it, sometimes twisting itself 
into a perfect corkscrew. And then by a law common to all leaf- 
<limbers, when it has once formed the spiral, it strengthens it greatly 
