4io Delf . — Transpiration in Succulent Plants. 
following pages this question will be discussed in its bearing on the problem 
of transpiration, firstly in relation to structural features, and secondly in 
relation to habitat. 
I. Transpiration in Relation to Structure. 
Succulent plants have a number of anatomical features in common 
which are undoubtedly connected with the process of transpiration. We 
shall consider the character of (a) the transpiring surface, (/3) the storage 
system, and (y) the conducting system, from the physiological point of view. 
(a) The Transpiring Surface. 
In plants of extremely dry habitats there is often a notable reduction 
in leaf surface, the stem itself taking on the bulk of the work of assimilation 
and transpiration ; there is therefore, presumably, a corresponding diminu- 
tion in the total water loss, although not necessarily in the actual rate of 
transpiration, per unit area of surface. Examples of this are seen in the 
thorny almost leafless stems of desert species of Euphorbia , and the equally 
thorny swollen stems of Opuntia , Echinocactus , and some other genera of 
the Cactaceae. The same tendency to eliminate the leaf as a distinct organ 
is seen in the halophytic genera Salicornia and Arthrocnemum , where the 
leaves are reduced to mere fleshy lobes almost entirely adnate to the stem. 
A large number of plants, however, have fleshy leaves or succulent stems 
without any such extreme reduction of surface, as, for example, the majority 
of epiphytic or halophytic plants ; these are often termed semi-succulents. 
It is commonly assumed that the percentage water content of a plant 
is in itself a criterion of its degree of succulence ; 1 but I have reserved this 
phrase to indicate the water content per unit area of surface. 2 In this sense 
the degree of succulence of a plant is of some interest for problems of 
transpiration, and a number of determinations from my own experiments 
are given in Table I. 
The method employed was that of finding the fresh weight of a leafy 
shoot, estimating the surface area, and drying at ioo°C. until a constant 
weight is obtained. The greatest difficulty lay in the determination of 
the surface areas. For large-leaved forms the outline of the leaf was 
carefully traced on paper ruled in millimetre squares ; when a number of 
leaves had to be traced in succession, they were kept in a damp chamber 
or floated on water until just before use, in order to minimize the shrinkage in 
surface due to loss of water in transpiration, which has been demonstrated to 
take place even before withering becomes perceptible. 3 In the case of small- 
leaved forms the procedure was varied to suit the particular type of leaf. 
1 Cp. Aubert. 
2 Delf,E.M. : Transpiration and Behaviour of Stomata in Halophytes. Annals of Botany, 191 1. 
3 Cp. Thoday, D. : Experimental Researches on Vegetable Assimilation and Respiration, V. 
Proc. Roy. Soc., B, vol. lxxxii, 1909. 
