BY SELF-ADAPTATION TO THE ENVIRONMENT. 253 
usually show no trace of them ; e. g. the whole section Hoarea of 
Pelargonium is bulbous.” This feature he attributes to the 
“« direct influence of the climate” *. 
Finally, with regard to the origin of water-storage tissues 
generally, I would suggest it to be primarily due to the accumu- 
lation of water within the plant in consequence of the arrest of 
transpiration. This latter function is impeded, in turn, by the 
formation of a thick and often waxy cuticle and a dense clothing 
of hair, as well as by the secretion of substances (such as tannin, 
gum, mucilage, resinous matter, salts, &c.) which thicken the 
water and so render it less capable of vaporization. But all 
these features, which thus bring about the very best structures to 
enable the plants to survive the injurious effects of the climate, 
are simply the direct outcome of the excessive heat and light 
coupled with the deficiency of water in the desert. 
XII. Secretions. 
These are of various kinds and appear to play an important 
part in arresting a loss of water. Thus, tannin is abundant 
in some desert plants, betraying itself by a yellowish appear- 
ance in the epidermal cells (Monsonia, Erodium, Astragalus, 
Tamarix, Linaria, Centaurea, &c.), as well as, of course, by 
the agency of iron salts. Dr. Volkens alludes to Warming’s 
suggestion that tannin, in connection with the hygroscopical 
capacity of acids, would afford a protection against desiccation Tf. 
In alluding to this supposed use of tannin, it may be mentioned 
that Sachs regards it as a waste product in metabolism f, though 
fungi when attacking oak-wood appear to consume it §. 
On the other hand, Mr. 8S. Le M. Moore remarks that “ tannic 
acid may have a more general relation to the turgescence of cells. 
Moreover, tannin is most likely used up in the lignification of the 
cell-wall” ||. As lignification is a prominent feature of desert 
plants, there may be perhaps more than a mere accidental 
coincidence. 
Another kind of secretion is that of strong scented ethereal 
oils, glandular hairs being a common feature on desert plants. 
Species of Artemisia are characteristic plants of the deserts of 
* “ Notes on the Regional Distribution of the Cape Flora,” Trans. Bot. Soc. 
Edinb. 1891, p. 241. 
+ Zur Flora der aegyptisch-arab. Wiiste, Skizze, p. 14. 
{ Physiology, &c., p. 828. § Op. cit. p. 388. 
| Journ, Linn. Soc., Bot. xxvii. p. 538. 
