440 Delf. — Transpiration in Succulent Plants . 
greater transpiration in succulents is not altogether clear, but is probably 
connected with the need for uninterrupted gaseous exchange, by means of 
the stomata, to compensate for the reduced surface area. 
Summary. 
In the course of this paper it has been shown that the chief structural 
peculiarities of succulent plants are connected with (i) the transpiring 
surface, and (2) the storage of water. 
Summing up for the characteristics of the transpiring surface, we 
see that succulent plants show a varying amount of reduction in leaf 
surface, and this may or may not be accompanied by arrangements 
which tend to diminish transpiration. Succulent plants of dry habitat 
have often either much cuticle or a waxy covering ; less frequently pro- 
tective hairs are found. In many cases water-storing tissue is found, and 
often the transpiration is considerable relative to the transpiring surface ; 
the stomata are often specially protected in plants of very dry habitats ; but 
in many Crassulaceae, succulent Chenopodiaceae, and halophytes, as well 
as many epiphytes, the guard cells are unprotected, and lie on a level with 
the epidermal cells. Many epiphytes absorb water over their whole surface, 
some also obtain water by means of aerial roots, which are provided with 
porous water-storing peripheral tissue, forming the so-called velamen. It 
seems probable that halophytes and desert plants with salt glands absorb 
water through the hygroscopic nature of the salts excreted ; and that other 
halophytes with a thin cuticle absorb water to a less extent by means 
of their leaves. 
Most succulent plants are characterized by the possession of definite 
aqueous tissue ; others have a great development of mesophyll in the 
leaves, and then the innermost layers tend to function as storage cells 
for water. The water stored may be required for long periods of drought 
as in all truly xerophytic succulent plants. In extreme cases, the whole 
plant may develop at the expense of the water stored, as in the corms 
of Sauromatum. On the other hand, the water stored may be of value in 
avoiding the evils of a fluctuating water supply, as in many rock plants ; 
or it may be a provision against temporary rapid transpiration, as in the 
Mangrove swamps of Ceylon, and perhaps, also, our ordinary British swamp 
halophytes. How far the aqueous tissue can be replenished from external 
sources, e. g. rain or dew or standing water, must be regarded at present as 
an open question ; but it is probable that this may at times take place. 
The formation of aqueous tissue seems intimately related, on the one hand, 
to the production of organic acids, owing to the influence of limited gaseous 
exchange on metabolism, and, on the other hand, to the presence of chlorides 
or sulphates in excess in the soil water. According to Holtermann, how- 
ever, these influences alone are not sufficient to account for all the peculiari- 
