434 Delf. — Transpiration in Succulent Plants. 
a kind of self-regulation, keeping the nature of the conducting tract in 
relation to the particular need of the plant . 1 
In the case of succulent plants we have little information as to the 
conducting tissue. Volkens, in his observations on plants of the Egyptian- 
Arabian desert, found that the conducting tissue of the shrubs and trees 
there was but poorly developed, and many of these would be succulent or 
semi- succulent plants either in stem or leaf. 
The experimental observations of Cannon , 2 however, at the desert 
laboratory at Tucson, do not bear out Volkens’s statement. A number of 
plants, including Fouquiera splendens , Covillea , and Zizy pints, were grown in 
the desert region, but were kept constantly irrigated with water for from 
two to four years. At the end of that time a careful comparison was made 
between the structure of the wood in irrigated and non-irrigated plants. It 
was found that in every case the irrigated plants grew much more luxuriantly 
and organized more woody tissue than the non-irrigated ; but they formed 
a relatively large number of non-conducting parenchymatous elements ; 
whereas in branches of the same diameter of the un watered plants larger 
and more numerous tracheal elements were found. Cannon concluded that 
the desert flora in these cases is a reaction to a minimum water supply, the 
relative transpiration being greater in the non-irrigated plants, although the 
total water loss would be less. 
In some epiphytes and many halophytes the aqueous tissue is provided 
with storage tracheides. These may be developed in connexion with the 
bundle system, and these usually occur as a group of isodiametric elements 
at the ends of the finest veins of the leaf. Often, however, storage tracheides 
are developed from the parenchymatous cells of the ground tissue, either 
singly as idioblasts, or as small groups of cells ; but in each case independent 
of the bundle system. These storage tracheides seem to play an important 
part in the distribution of the water in fleshy stems and leaves. They 
occur always near a source of water, and are usually also in connexion with 
the assimilating cells, which they probably keep supplied with water. 
There appears to be no direct evidence that these tracheides can exercise 
a distributive function, but in the case of many species of Salicornia , 
Dr. E. de Fraine 3 tells me that their disposition in the stem would lend 
support to this view. They appear to be most numerous near the periphery 
of the fleshiest region of each internode, and they are more numerous 
in the flowering than in the vegetative shoot. In Salicornia the storage 
tracheides are unlignified, but they are spirally thickened with cellulose, and, 
according to this observer, retain their shape even when empty of water ; 
1 Haberlandt, Dr. G. : Physiologische Pflanzenanatomie, 1896, p. 296. 
2 Cannon : W ater-conducting Systems of Desert Plants. Bot. Gaz., 1905. 
3 Moss, de Fraine, and Salisbury : On the genus Salicornia (anatomical section). Linn. Soc. 
Trans, (in the press). 
