236 ABSORPTION-CELLS ON LEAVES. 



situations crystals of salt are sometimes to be seen separated out from the soil, and 

 lying as a white efflorescence upon the ground, it used formerly to be believed that 

 the salt incrusting leaves and stems was derived, not from the plants in question, 

 but from the soil around, and had only spread from there over the various plant- 

 members. But this is not the case. As a matter of fact, the salt observed on the 

 leaves and stems of Frankenia, Reaumuria, Hypericopsis p)ersica, and a few species 

 of Tamarix and Statice, is produced from the substance of the leaves. It is excreted 

 in just the same way as the crust of lime, above described, is from the leaves of 

 saxifrages. To the naked eye the surfaces of the leaves in all the plants enumerated 

 have a punctate appearance. On closer inspection, it is evident that, corresponding 

 to each dot, there is a little cavity, the deepest part of which is constructed of cells 

 with extremely delicate external walls. In quite young leaves only a single thin- 

 walled cell of the kind is to be seen at the bottom of each shallow depression. But 

 this divides, and, by the time the leaf is full-grown, from two to four cells are seen 

 to have arisen by division of the one cell. Stomata are, in addition, intercalated in 

 the membrane in the neighbourhood of these thin- walled cells, and, in the rainy 

 season, when there is no lack of water in the habitats of the plants in question, a 

 watery juice, containing a large amount of salts in solution, exudes from these 

 stomata. The saline solution soaks over the whole surface of the leaf, and in a dry 

 atmosphere crystals form from it and adhere to the leaf in the form of little gland- 

 like patches or continuous crusts. 



If these tamarisks, frankenias, and reaumurias are observed during: a rainless 

 season, the crystals of salt are seen under the noon-day sun glittering on the leaves 

 and stems, and may be detached in the form of a fine crystalline powder. But if 

 the same place is visited after a clear night, no trace of crystals is to be seen; the 

 little leaflets have a green appearance, but they are covered with a liquid with a 

 bitter salt taste,-"- and are damp and greasy to the touch. The crystals have 

 attracted moisture from the air during the night, and have deliquesced, and the 

 saline solution not only covers the whole of the leaf, but also fills the little cavities 

 visible as dots to the naked eye. The thin-walled cells at the bottom of the cavi- 

 ties differ from the rest of the epidermal cells and the guard-cells of the stomata, in 

 that they are susceptible of being wetted, and they may act as absorption-cells, and 

 allow the water, attracted by the salts from the air, to pass through their thin 

 walls into the interior of the leaves. 



When the air dries under the rising sun, crystals are again formed from the 

 solution of salts, and, covering the leaves once more in the form of crusts, fill up the 

 depressions and protect the plants during the hot hours of the day from excessive 

 evaporation. Whilst, therefore, in the dewy night these plants are indebted to 

 their salt crusts for water, they are in the day-time preserved from desiccation by 

 the action of the same contrivance. 



^ The salt incrustations which were removed from plants of Franhenia hispida, collected on a Persian salt-steppe, 

 consisted principally of common salt (chloride of sodium). They contained in smaller quantities, gypsum, mag- 

 nesium sulphate, calcium chloride, and magnesium chloride. 



