234 ABSORPTION-CELLS ON LEAVES. 



(cf. fig. 26^). The bottom of each depression is made up of cells with very thin 

 external walls, and the function of these cells is to suck up the water that flows 

 into the cavity. It is obvious that the absorbed water passes thence into tho 

 enlarged extremities of the branches of the vascular bundles, and may then be 

 conducted to other parts of the leaf. Seeing that all these saxifrages have their 

 habitat in crevices of rocks on sunny declivities, they are much exposed to 

 desiccation in times of drought. The epidermal cells of the medial area and those 

 of the extreme edge are no doubt protected by a very thick cuticle (see fig. 55^); 

 but in the case of the thin-walled cells at the bottom of the depression there is the 

 danger of as much or even more water escaping through them, in the form of vapour, 

 than has been previously taken in during the prevalence of rain. 



In order to prevent this loss of moisture recourse is had to a very remarkable 

 contrivance for closing the cavity, viz., an incrustation of carbonate of lime. In 

 many saxifrages this crust covers the whole face of the leaf, in others only the 

 margin, or the spot where the depression occurs. In the latter case it looks like a lid 

 over the cavity. At that spot the crust is always thickened, and sometimes it forms 

 a regular stopper which fills up the entire cavity. It rests upon the epidermis of 

 the leaf, but is not adnata thereto, and may be removed with a needle. When a 

 leaf is bent the crust is ruptured and breaks up into irregular plates and scales, 

 and a strong gust of wind would then easily strip off the fragments and blow them 

 away. In species subject to this danger, as, for instance, Saxifraga Aizoon, in 

 which the resetted leaves curl strongly upwards and inwards in dry weather, the 

 crust of lime is held fast by peculiar plugs which arise from individual epidermal 

 cells projecting above the rest in the form of papillae (see fig. 55^). These plugs 

 are found principally on the side walls of the cavities, but are also scattered every- 

 where on the epidermis of the margin of the leaf. They are so incrusted with the 

 lime that the latter cannot easily fall off, and a comparatively strong pressure must 

 be applied with the needle to detach it from the substratum. The calcium carbonate 

 of which these crusts consist is excreted in solution by the plant from pores occur- 

 ring at the bottom of the depressions. The pores are constructed like ordinary 

 stomata, but are, as a rule, somewhat bigger, and it is not improbable that, when 

 once the lime crust has formed from the excreted solution, they take part in the 

 function of transpiration. 



There is scarcely any need for further explanation of the manner in which the 

 apparatus here described acts. When rain or dew falls on a saxifrage leaf the 

 whole upper surface is moistened directly, whilst the water soaks under the crust 

 of lime, and, diffusing itself there, fills in a moment the depressions, and is taken 

 up by the absorption-cells situated at the bottom of the latter. The calcareous 

 stopper imbedded in each cavity is only upheaved by this process to a trifling 

 extent. In dry weather the crust is appressed closely to the epidermal cells, and 

 the stopper descends again and impedes the evaporation of water from the thin- 

 walled cells within the cavities. 



The absorptive organs on the leaves of Acantholimon, Goniolimon, and a few 



