238 ABSORPTION-CELLS ON LEAVES. 
The reason for the contrivance just described being exhibited especially by the 
marginal teeth of the leaf, lies in the fact that dew is deposited particularly at those 
spots. If one looks at the leaves of the dwarf almond and plum trees in the 
steppe-districts, after clear summer nights, one finds a dewdrop suspended to every 
tooth on the margins; but by noon all the teeth are dry again and protected from 
loss of water by the coat of varnish. Moreover, not steppe-plants alone, but very 
many plants which grow in poor sandy soil on the banks of streams and rivers, 
exhibit this contrivance for the direct absorption of water from the atmosphere. 
Instances are afforded by the Sweet Willow, the Crack-willow, Poplars, the Guelder- 
rose, the Bird-cherry, and many others. It is at once evident that this contrivance 
is observed chiefly on the leaves of trees, shrubs, and tall herbs, whilst incrustations 
of lime occur only on shorter plants with rosulate leaves spread out on the ground, 
or with rigid acicular leaf-structures. The grounds of this distinction may well 
reside in the fact that the weight of a crust of lime is many times as great as that 
of the dry film of varnish. A load capable of being borne without hazard by the 
leaves of a Statice plant, they being spread out on the ground, or by the rosettes of 
Saxifraga Aizoon, would be unfit for the leaves of a Cherry or Apricot tree, or for 
those of the Sweet Willow, or the Crack-willow; indeed the branches of these trees 
would break down under the burden if their leaves were incrusted with lime. 
In many cases only a few of the marginal teeth of the leaf are transformed into 
absorbent apparatus, and special contrivances then always exist to convey rain and 
dew to those teeth. The Aspen (Populus tremula) serves as a very good example 
of this. This tree has, as is generally known, two kinds of leaves. Those arising 
from the branches of the crown have long petioles and laminz of roundish outline and 
with somewhat sinuate margins; those which are borne by the radical shoots have 
shorter stalks and larger sub-triangular lamin sloping outwards; and the whole 
leaf is so placed and its margin so curved as to oblige the rain which strikes the 
upper surface in its descent to flow down towards the petiole (see fig. 551). Now, 
situated exactly on the boundary of lamina and petiole are two cup-shaped structures 
(fig. 55*) originating from the lowest teeth of the leaf, and so arranged that every 
drop of rain descending from the lamina must encounter their shallow cavities and 
fill them with water. These cups are brown in colour and the size of a grain of 
millet; and the cells of their epidermis are furnished with a thick cuticle. Only 
the cells lining the shallow depression of each cup have thin walls, and they excrete 
a sweet-tasting, slimy, resinous substance which in dry weather films over the 
cavity like a varnish, and protects, at all events, the cells lying beneath it against 
an injurious desiccation. When, however, this coat is itself in contact with water 
it swells up, and the moisture is then absorbed by the cells in the pit-like depression 
and is transmitted to the vessels running underneath the cups (see fig. 55°). 
A number of tall herbs, principally of the group of Composite, have, like the 
Aspen, leaf-teeth which are developed at the part where petiole and lamina join and 
act as organs of absorption. In some, besides, the margin of the green lamina 
extends in the form of a narrow ridge down the pale canaliculate petiole; and, when 
