412 Delf. — Transpiration in Succulent Plants . 
The plants fall roughly into three groups which may be termed 
‘succulent’, ‘semi-succulent’, and ‘ mesophytic *, respectively. The plants 
belonging to the first group possess from 5 to 12 grm. water content to 
every square decimetre of surface ; tcfthis class belong, probably, the xero- 
phytic succulent Cactaceae (Opuntia, &c.). The same degree of succulence 
may be attained by large and small leaved forms, such as Mesembryan themum 
ed2ile and Suae da maritima ; by adnation of the reduced leaf (as in Salicornia, 
and to a less extent in Sals old), or by the development of aqueous tissue in 
each separate leaf, as in Arenaria peploides or PI ant ago maritima. The 
plants belonging to the second group comprise most of our British halo- 
phytic plants, many rock plants, and would probably also include many sub- 
tropical or tropical epiphytes, and plants typical of the Mangrove swamps. 
These have from about 2 to 5 grm. water content per sq. dm. surface. The 
third group includes only typically mesophytic plants, such as Saponaria 
and Mercurialise and perhaps also a few exceptionally thin-leaved halophytes 
like Statice, in which the aqueous tissue is limited to the region of the 
midrib, and in which the epidermis is provided with small excretory glands. 
Frankenia and Tamarix, two other somewhat thin-leaved halophytic genera, 
at least in some of their species, are also provided with epidermal glands, 
and it is possible that the excretion of salt which is said to take place 
through these glands may account for the absence of the succulent habit. 
Whether reduced or not, the transpiring surface is protected by 
a definite tissue, the epidermis, which may be more or less cuticularized. 
Haberlandt 1 describes an experiment in which he demonstrates the pro- 
tective power of the epidermis in succulent plants. Two pieces of an Aloe 
leaf were covered with wax, except for a definite area, equal in both cases. 
The epidermis was carefully stripped from the exposed surface of one piece, 
but was left intact on the other ; both pieces were then exposed to the air 
for three hours. The pieces were then weighed and the stripped leaf was 
found to have lost 524 mg., whilst the uninjured piece had lost only 2 mg. 
In this case the epidermis has a well developed cuticle, but there is, no 
doubt, a considerable amount of protection derived from even a slightly 
cuticularized epidermis. 
Many succulent xerophytes have a thick cuticle and relatively little 
water loss, as the work of both Schimper 2 and Volkens 3 shows. Holtermann 
also found some Cactaceae with thickly cuticularized epidermis in which 
the transpiration was almost imperceptible. In alpine plants, according to 
Leist and Bonnier, 4 the outer wall of the epidermis is more strongly 
1 Haberlandt : Physiologische Anatomie, p. 96. 
2 Schimper, A. F. W. : Ueber Schutzmittel des Laubes gegen Transpiration, besonders in der 
Flora Javas. Monatsber. der Akad. Wiss. in Berlin, xl, 1890. 
3 V olkens, G. : Zur Flora der agyptisch-arabischen Wiiste, auf Grundlage anatomisch-physio- 
logischer Forschungen, 1887. 
4 Leist and Bonnier : Quoted by Dr. A. Burgerstein, in * Die Transpiration der Pflanzen ’, 1904. 
