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. 54 ^ ). The simple 

 hairs and peltate groups of cells developed in the grooves and ducts (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 Gentianese — most conspicuously in the large-flowered Dwarf Gentian 

 (Gentiana acaulis) — the decussate pairs of radical leaves form a loose rosette (see 

 fig. 52^). 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 pufled 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 Bromeliacese 

 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-Idcea) 

 Httle 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 



