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ABSORPTION-CELLS ON LEAVES. 231 
shaped structures, to collect and absorb the water; and the construction of the 
protective apparatus, which prevents too rapid evaporation into the air of water 
that has once flowed into the depressions, is as various as the form of the depressions 
themselves. A short account of the most striking of these structures will now be 
given. 
Such water-collecting grooves as are closed, so as to form ducts, occur principally 
in petioles and in the rachises of compound leaves. For instance, in the Ash the 
leaf rachis, from which the leaflets arise, is furnished with a groove on its upper 
surface. Owing to the fact that the edges of this groove, which are strengthened 
by a so-called collenchymatous tissue, are bent up and curved over the groove, a 
duct or conduit pipe is produced, and this duct only gapes open at the places where 
the leaflets are inserted upon the rachis, and where, therefore, the drops of rain to 
which the leaflets are exposed flow off into the groove (see fig. 541). The simple 
hairs and peltate groups of cells developed in the grooves and duets (fig. 54° and 
54°) are not merely transiently moistened, but inasmuch as the water is retained 
there for several days after a fall of rain, they are during that time immersed in a 
regular bath of water, and are able to absorb the moisture very gradually. 
In many Gentianeze—most conspicuously in the large-flowered Dwarf Gentian 
(Gentiana acaulis)—the decussate pairs of radical leaves form a loose rosette (see 
fig. 521). The larger dark-green blade of each leaf is flat and even, and only the 
pale-coloured base is fashioned into a groove. This groove is made deeper by the 
tissue of the leaf being puffed up round it, and as all the leaves of the rosette 
arise close together, the groove of each leaf is covered by the lamina above it. 
The rain or dew accumulated from the blade remains standing in this concealed 
nook for some time without evaporating, so that absorptive apparatus with the 
power of taking up water has plenty of time for the purpose. In this case the 
absorptive apparatus is in the hindmost extremity of the groove, and consists of 
long, club-shaped structures composed of extremely thin-walled cells (see fig. 54*), 
and these act so energetically that if leaves are cut off and left to fade, and if the 
cut surfaces are stopped with sealing-wax, and the whole then bathed with rain- 
water, they take up in twenty-four hours about 40 per cent of their weight of 
water. A similar phenomenon occurs in the case of a number of Bromeliacez 
which adhere by a few roots to the bark of trees in the tropics, and have grooved 
rosetted leaves, the latter covering one another, and being arranged in such a 
manner as to form a regular system of cisterns. At the bottom of each cistern 
there are special groups of thin-walled cells which suck up any water that flows in 
when rain falls. 
On the under surface of the leaves of the Cow-berry (Vaccinium Vitis-Idea) 
little depressions are formed, and in the middle of each depression there is a club- 
shaped structure composed of small thin-walled cells, which contain slimy, viscid 
substances and act as absorbent organs. The rain which falls upon the upper 
surface of the leaf gets drawn over the edges on to the under surface, fills the small 
depressions occurring there, and is taken up by the absorptive apparatus. A 
