THE CONDITIONS FOR THE DISTRIBUTION OF PLANTS. 108 
Most Indian trees and shrubs have a glossy green foliage, e.g., the 
leaves are coriaceous or have a strong culicula (the outer coat of 
the epidermis). he protection of the mesophyll, or the inner parts 
of the leaves, is not only due to the close texture of the cuticula, but 
also to its glossy nature, by which quality the rays of light and 
heat are reflected. ‘Trees and shrubs with tender foliage as a rule 
shed their leaves in the dry season, unless these are hairy or 
protected by other means. Among these I shall mention the 
periodical movements of leaves, especially the regular folding up of 
the leaves at nightfall, as we meet with in many Acacia, Mimosa, 
Albizzia, Pithecolobium, and which is frequently called the sleep of 
plants.* his peculiar phenomenon is probably a very necessary 
provision which, though caused by the absence oflight, is actually 
intended for exposing thé underside of the leaflets to the full benefit 
of absorbing moisture during the night. Suclr leaves have as a 
rule channelled petioles, which are probably meant for retaining 
the dew. 
The leaves of other plants, as of the curious forms of Cactaceous 
plants and leafless Huphorbias, are frequently ‘transformed into 
spines or scales as in Opuntia Dillent and Euphorbia Tirucalli, im 
order to reduce their evaporation and to enable the plants to with- 
stand heat and drought. All sueculent plants, of which the common 
Aloe, so frequently seen suspended in front of shops im the bazaar, 
is a good example, are similarly well adapted to flourish im the 
driest localities. 
Tubers, bulbs or rhizomes, which in India are represented by 
numerous species of Aracew, Zingiberacer, Amaryllidee, Ferns, &c., 
are other structures that preserve the plants during the dry season, 
when their annual top dies away. The pseudo-bulbs, fleshy stems 
and quast-parasitic roots of Orchids are similar contrivances. 
Finally the milky juice of plants, so characteristic in the genus 
Vuphorbia, in many Ficus, and several Asclepiads, the secretions of 
wae or other vegetable fats, and the resinous substances found on 
many plants, may also be regarded as protection against heat, but 
more probably. as an adaptation for sudden changes of tempera- 
ture, 
Next to severe cold, excessive heat and drought, the wird is the 
most active of climatic agencies. Though in many respects very 
* Since I wrote this, | have observed that the leaves of Cassia marginata expose 
their upper surfaces during the night, but in this case the under side is very hairy. 
