Delf,— Transpiration in Succulent Plants . 423 
their whole surface ; but it has long been a matter of dispute how far this 
power is of importance to the plant in nature. The work of Wille 1 and 
others shows that the proportion of water thus absorbed, relative to that 
taken up by the roots, is mostly so small that the surface absorption can be 
of but little meaning biologically. 
However, there is no doubt that such surface absorption may be of the 
utmost value ; rootless epiphytes, indeed, depend wholly on this method for 
their water supply, and in such cases adaptive mechanisms for collection and 
storage of water are often found, as, for example, the spoon-shaped leaves of 
some Bromeliaceae and the absorbing hairs of Usnea tillandsioides. 
Aerial roots form additional organs of absorption in many epiphytes, 
especially in the epiphytic Aroids and tropical orchids. These are fleshy, 
greenish roots which stand out from the surface of the plant and are covered 
with a parchment-like layer of aqueous tissue, several cells deep. The cells 
of the aqueous tissue are usually thin walled, isodiametric, and lined with 
a delicate spiral of cellulose ; often the walls are perforated, so that when 
water reaches the surface it is sucked in rapidly by capillary attraction. It 
has even been suggested by Schimper and Goebel 2 that the aqueous tissue, 
or velamen, as it is here frequently termed, has the power to absorb water 
vapour and nutritive gaseous compounds, such as ammonia, from the 
air ; a very probable suggestion, although not yet substantiated by 
actual facts. 
A number of other plants, however, show a considerable power of 
surface absorption, the value of which has been perhaps under-estimated. 
The papillate hairs of certain species of Mesembryanthemum , to which 
reference has already been made, are of some interest in this connexion, for 
they appear to have a marked power of absorption in addition to their 
capacity for retaining and storing water. Some cuttings of M. edule and 
M. crystallinum had been received by me from Cornwall, where both plants 
grow freely in a semi-wild state. They had been packed loosely in a card- 
board box and were obviously suffering from want of water when unpacked. 
M. edule is a glabrous species with somewhat thickened cuticle and slightly 
sunken stomata. M. crystallinum has papillate leaves with little or no 
cuticle, and small unprotected stomata, and it therefore seemed worth while 
to compare their power of surface absorption. A shoot of each was chosen 
with about the same number of healthy uninjured leaves ; they were detached 
and weighed accurately after the cut end of each had been sealed up with an 
adhesive wax mixture. The shoots were then immersed in water, and dried 
and weighed at intervals. The results given in Table V show that M. crys- 
tallinum has an absorption from three to seven times as great as M. edule . 
Both these species live in dry, rocky places where there is a limited 
1 Burgerstein, Dr. A. : Die Transpiration der Pflanzen, p. 232. 
8 Haberlandt, Dr. G. : Die physiologische Pflanzenanatomie, 1896, p. 201. 
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