Delf,— Transpiration in Succulent Plants . 421 
su Pply is available, whether from root absorption or from rain or dew, the 
cells expand again and become again water reservoirs. In the leaves of the 
epiphytic Peperomia incana the delicate cell-walls of the aqueous tissue are 
strengthened at the corners, where they abut on the palisade tissue, with 
a horn-like substance, so that a firm framework is interposed between the 
mesophyll cells and the aqueous tissue when the latter collapses in dearth 
of water . 1 Occasionally the aqueous tissue is formed of deep, narrow, 
palisade-like cells, as in Carappa moluccensis and Rhizophora mucronata . 
Very often the amount of aqueous tissue increases in volume as the leaf 
grows in age, and this is accomplished by an often considerable increase in 
the size of the individual cells. This increase in storage capacity as the 
leaf attains its full size is of obvious importance in the economy of the 
plant. 
Peripheral aqueous tissue is very characteristic of the leaves of 
epiphytes and of Mangroves. According to Holtermann 2 many epiphytic 
species of Ficus and Peperomia show well-developed aqueous tissue, whereas 
terrestrial species of the same genera may be entirely lacking in this respect. 
If the plant is growing in a very exposed, sunny place, the aqueous tissue 
will be protected on the outside from excessive water loss, as in Ficus 
tomentosa , by the formation of cuticle and hairs ; but in epiphytes of damp 
regions, and constantly in Mangroves and plants of the Solfataras, the 
aqueous tissue is little protected, and at certain times may show enormous 
loss of water in nature. 
Volkens 3 has described various species of A triplex with water-storing 
hairs, and also some desert species of M esembryanthemum ; to these refer- 
ence has already been made . 4 There is no doubt, as Haberlandt insists , 6 
that the presence of mucilage in the cell-sap of the aqueous tissue greatly 
increases its power of resistance to desiccation, even in otherwise little pro- 
tected leaves, such as those of M esembryanthemum crystallinum . The 
mucilage may be distributed throughout the whole of the aqueous tissue, as 
in Agave , or localized in special cells, as in Rhizophora mucronata ; or 
it may be associated with storage tracheides, as in the leaves of Loranthus 
europaeus. 
Most European succulent plants have an internal but no peripheral 
aqueous tissue ; however, the colourless cells of the epidermis may them- 
selves function to some extent as water reservoirs, as may be observed by 
actual experiment. For example, in one case observed by me, a leaf of 
Aster Tripolium was picked and at once placed in water. Owing, perhaps, 
to the dry air of the laboratory, both upper and lower epidermis when 
1 Haberlandt, Dr. G. : Physiologische Pflanzenanatomie, 1896, p. 347. 
2 Holtermann, Dr. C. : Der Einfluss des Klimas auf den Bau der Pflanzengewebe, 1907. 
3 Volkens : Zur Flora der agyptisch-arabischen Wuste, 1886. 
4 See p. 416 in present paper. 
6 Haberlandt, Dr. G. : Physiologische Pflanzenanatomie, 1896, p. 352. 
F f 
