ABSORPTION-CELLS ON LEAVES. 235 



other Plumbagineae, resemble in an extraordinary degree those pertaining to saxi- 

 frages. The depressions are here found uniformly distributed over the entire sur- 

 face of a leaf, and when they are closed by a crust or scale composed of calcium 

 carbonate, the leaves are dotted with white spots, as may be seen in the drawing 

 of a leaf of Acantholimon Senganense given in fig. 55 4 . Upon the calcareous scale 

 being removed, a little cavity is revealed beneath, and one observes that the floor of 

 this cavity is composed of from four to eight cells, separated by radial partition- 

 walls, and with exceedingly thin and delicate outer walls. The other epidermal 

 cells adjoining the cavity are, on the contrary, always furnished with a thick cuticle 

 (see fig. 55 5 ). Whenever water is being copiously supplied to the roots, and the 

 turgidity of the cells in the leaves is great, the cells forming the floor of the cavity 

 excrete bicarbonate of lime in solution. Part of the carbonic acid escapes into the 

 air, and the insoluble mono-carbonate of lime in the water then forms a crust, which 

 fills and covers the cavity, and often even spreads over the whole leaf, constituting a 

 coherent calcareous coat. 



All Plumbagineaa which exhibit this contrivance that is to say, the various 

 species of Acantholimon, Goniolimon, and Statice inhabit steppes and deserts, 

 where in summer no rain falls for months together, and the soil becomes dry to a con- 

 siderable depth, so that extremely little water is available for the roots. Although 

 the rigid leaves are protected by a thick cuticle, and by crusts and scales of lime 

 against excessive evaporation of their aqueous contents, still it is difficult to avoid 

 some slight loss of water, especially when the noon-day sun beats down upon the 

 steppe, and, owing to the extremely arid nature of the soil, it is scarcely possible to 

 replace this loss, however small it may be, by absorption from the earth on the part 

 of the suction-cells on the roots. All the more welcome to plants of the kind is the 

 dew which sometimes falls copiously on steppes and in deserts in the course of the 

 night; it wets the rigid leaves, and, soaking immediately underneath the crusts and 

 scales of lime to the thin-walled cells at the bottom of the cavities, is absorbed with 

 avidity by them. When drought returns with the day, the scales of lime close 

 tightly down like lids on the epidermis beneath, and, so far as possible, prevent 

 evaporation. In particular, they impede the exhalation of water from the thin- 

 walled cells at the bottom of the cavities a loss which would otherwise be quite 

 inevitable, and would be followed by a rapid desiccation of the entire plant. To 

 prevent the calcareous lids from dropping off, there are either, as in Saxifraga 

 Aizoon, papilliform or conical projections from cells in the immediate vicinity of 

 the cavities, which projections often have hooked ends and confine the crust of 

 lime, or else each cavity is somewhat contracted at the top and enlarged below, so 

 that the lime stopper, being shaped according to the contour of the cavity, cannot 

 fall out. 



A significance similar to that attributed to calcium carbonate excretions belongs 

 also to the saline crusts which are found covering the leaves of a few plants grow- 

 ing on the arid ground of steppes and deserts in the neighbourhood of salt lakes 

 and on the dry tracts of land near the seashore. Owing to the fact that in these 



