228 REV. G. HENSLOW ON THE ORIGIN OF PLANT-STRUCTURES 
IV. Foliage of Desert Plants. 
Another result of the deficiency of water is the small size of 
the leaves of desert plants, thereby lessening the surface of the 
transpiring organs; or else they are suppressed almost, if not 
quite, altogether, as in Retama, Anabasis, Ephedra, and Tamaria. 
Many plants produce moderate-sized leaves iu the early spring as 
soon as the rainy season commences, but none or very small ones 
later on, as Zilla, Alhagi, Statice, &c. The inrolled margins of 
the leaves, which make them assume the form of a more or less 
closed cylinder, is another common contrivance. : 
Now while this reduction of surface is beneficial by lessening 
transpiration, we must remember that it is simply the result of 
drought. This is proved in several ways: first, one and the 
same plant will produce much larger leaves in March or April, 
when a good supply of water is at hand, but minute leaves in 
June, when the supply is deficient ; secondly, if the same plant 
be grown in the Nile Valley it ceases to produce the smaller 
summer foliage and resembles the ordinary herbaceous leaves of 
temperate climates. Thus Salvia lanigera growing in the Delta 
has flat leaves, eight inches in length; but when in the Desert 
they are only about two and a half inches long, with the margins 
inrolled. A similar variability is a common phenomenon and is 
well known; for numerous instances might be given of leaves 
varying in form and structure on the same plant or on different 
individuals, according as they develop at different times or 
under different circumstances—or, again, if the plant have been 
transplanted, say, from a hot toa cold, or from a dry to a wet 
locality ; or, again, from a low to a high altitude or vice versd. 
Mr. Groom records a case of an orchid, Renanthera albescens, 
which naturally scrambles over plants growing on hot open 
sandy heaths. The specimen had been transferred to the botanic 
garden, Singapore, where it was growing under the shade of a 
well-foliaged tree. As might be expected, the form and 
anatomical details of the new leaves became much altered. Mr. 
Groom gives a series of comparisons showing how the cuticle 
decreased in thickness, while the dimension of the leaf increased 
in length but was diminished in thickness, &c.* 
Mr. Scott Elliot t has independently arrived at a very similar 
* «Annals of Botany,’ vii. p. 152. 
+ “ Notes on the Regional Distribution of the Cape Flora,” Trans. Bot. Soe. 
Edinb, 1891, p. 241. 
