426 Delf. — Transpiration in Succulent Plants . 
found that the rate of transpiration increased steadily from the young 
shoot to the old, until the quantity of mucilage and gums began to become 
important. From that time the transpiration steadily fell off, until in the 
adult segments it was almost negligible. 
Aubert’s work further shows that the presence of organic acids is 
intimately connected with the succulence of the plant; thus a number of 
different species of Sedum arranged in order of their acid content per gramme 
fresh weight were found also to be arranged roughly in order of their 
fleshiness. This is intelligible when we remember that both organic acids 
and the sugars which are derived from them in metabolism are osmotically 
active substances, and would exert a high attraction for water. 
In the Cactaceae the organic acid is mostly present as salts of malic 
acid ; in the Crassulaceae mostly as free or combined isomalic acid, together 
with traces of tartaric acid, whilst the species of Mesembryanthemum ex- 
amined gave abundant oxalates together with traces of mineral acids. The 
observations of Wolff 1 and others show that in halophytes malic acid is 
present in small amount, in addition to chlorides. 
It has been suggested by Diels 2 that it is the presence of organic acids 
in halophytes which prevents the harmful accumulation of salts in the 
aqueous tissue of those types which do not possess secretory glands. The 
succulent habit is then regarded as connected with the formation of organic 
acids in restricted respiration, rather than with the biologic necessity of 
reducing transpiration, in order to avoid harmful accumulation of salts 
brought up by the ascending sap. 
This theory was based upon a series of experiments, in which it was 
found that if a normal halophytic plant, such as Cakile maritima or Sali- 
cornia , was kept for some time in distilled water, or in a very dilute solution 
of chlorides, a gradual diminution in the percentage chloride content of the 
plant took place. Since the plants were, as far as possible, uninjured, the 
whole root system having been transferred to the solution, the inference was 
that the chlorides had in some way interacted with the organic acids and 
passed out from the plant in a form not thus far detected experimentally. 
Unfortunately, all the calculations made were reckoned on equal initial 
fresh weights, no allowance having been made for changes in water content 
during the course of the experiment, owing to the effect of the transference 
to water or salts, or owing to changes in the physical conditions affecting 
the rate of transpiration. In a critical account of Diels’s theory, Benecke 3 
drew attention to these omissions, and further recorded some experiments 
made by himself, following Diels’s method as nearly as possible, but ob- 
1 Wolff: Quoted by Burgerstein, loc. cit. 
2 Diels: Die Entchlorung und der Stoffwechsel der Halophyten. Pringsheim’s Jahrbiicher, 1906. 
3 Benecke, W. : Ueber die Dielsche Lehre von der Entchlorung der Halophyten. Pringsheim’s 
Jahrbiicher, 1901. 
