Delf. — Transpiration in Succulent Plants. 425 
which clothe the surface of many Chenopodiaceous plants may be absorbing, 
but this has not yet been experimentally demonstrated. 
1 1 have suggested that species of Saliconia and Aster Tripolium, which 
are very sensitive to dry air and which are subject to a variety of conditions 
physically, may derive real benefit from their well-marked power of surface 
absorption. The same suggestion was made independently by Miss A. C. 
Halkett, 2 whose experiments bear this out to some extent. 
(/3) The Water-storing System. 
We come now to the question of the system of water-storing tissues 
which are essentially characteristic of succulent plants. We have already 
seen that many epiphytes and mangroves have a superficial storage tissue, 
which perhaps also affords protection to the chloroplasts from the intense 
sunlight ; many of these also have internal water-storing tissue, so that the 
assimilating cells are covered both above and below by aqueous tissue, as 
in many species of Peperomia. In fleshy leaves the water-storing tissue 
often gradates insensibly into spongy mesophyll cells with very scattered 
and pale chloroplasts ; it is, however, sometimes localized in thick fleshy 
petioles. In stems the aqueous tissue is often located in the cortex, either 
completely encircled by the chlorophyll-containing cells, as in Salicornia , 
or interrupting the assimilating tissue in places, as in Salsola , where the 
stem shows streaks of transparent tissue between the chlorophyllous tissue ; 
the epidermis above the transparent streaks has no stomata. Water is also 
stored in tubers or corms, as in epiphytic orchids, and in the species of 
Sauromatum popularly known as ( Monarch of the East \ In these cases 
the plant can live for long simply on this reserve, and in the last case the 
whole flowering spadix and the single large leaf are produced at the expense 
of the corm, which shrinks greatly as development proceeds. 
In many cases the aqueous tissue contains a highly acid or highly 
saline cell-sap, which produces a high osmotic pressure internally. This 
may serve to facilitate surface absorption in plants, such as Mesembryanthe- 
mum cvystallinum and Salicornia , but it certainly tends to retard evaporation, 
for water evaporates less readily from stronger than from weaker solutions ; 
thus Aubert 3 found that in Sedum dendroideum the acid content increases as 
the young leaf becomes adult, and those leaves which had the maximum 
acid content had also the least transpiration. In many cases, in addition to 
the acid cell-sap, mucilage passages or cells occur, as in the Cactaceae, 
which are even more retentive of water ; for example, with Opuntia , Aubert 
1 Delf, E. M. : Transpiration and Stomatal Behaviour in Halophytes. New Phytologist, 1911. 
2 Halkett, A. C. : Some Experiments on Absorption by the Aerial Parts of Certain’Salt-marsh 
Plants. New Phytologist, 1 91 1. 
3 Aubert, E. : Turgescence et transpiration des plantes grasses. Annales des Sciences Nat. ; 
Bot, 1892. 
