MIXED CSOP Oi' UtfiroHM AGE. 61 



species but possess the same spread of crown, that one williresist 

 better which has the more open crown. Again,of two species in 

 question, one may be deep-rooted, while the other has its roots 

 more or less superficial and is consequently less able to resist vio- 

 lent movements of the atmosphere. Thus sal is comparatively 

 rarely blown down by the wind. So also where storms and gales 

 are frequent, as on ridges, spurs, peaks, in narrow gorges, or on 

 the sea coast, the strong-rooted individuals will be the ultimate 

 survivors, and species with weak superficial roots will have to re- 

 treat to sheltered localities. After a deep and strong root-system, 

 strength and elasticity of the wood are the most important quali- 

 ties a species can possess as a safeguard against violent winds. 

 Lastly, a species that is leafless when a strong wind is blowing is 

 of course less Hkely to be injured than one in leaf ; hence in ex- 

 posed windy situations deciduous trees, as a rule, thrive better 

 than evergreens. In considering the effect of a wind we must not 

 forget that accompanying rain adds very considerably to its violence. 

 Species that are in leaf, or bring out their leaves, while periodical 

 hot or chilly winds are blowing, will generally suffer more than 

 those that are leafless at the time. Thus the hot winds Hmit the 

 expansion to the south and west of the Sub-Himalayan sal belt. 

 Species with leaves or other transpiring organs of firm texture, or 

 covered with a thick and waxy epidermis, such as Prosopis spicigera. 

 Euphorbias, the tamarisks, Capparis aphylla, &c., are able to offer 

 a more or less successful resistance to the hottest and driest winds. 

 XI. Gradient. — One of the principal effects of gradient, when 

 there is no impeding cause, is to drain off the moisture of the soil. 

 Trees like sissu, the tamarisks, &c., require a great deal of conti- 

 nually moving water in the soil, and they can hence grow only 

 where the ground slopes very moderately but the soil transmits 

 water freely. Other species, like sal, require a less quantity of 

 water and a less perfect, though still free, drainage ; hence their 

 affecting only steep or rolling hill sides, except when, as in the 

 Bhabar, the soil itself or at least the subsoil lets water pass through 

 it as through a sieve. A third class, like teak, requires still less 

 moisture in the soil, but does not object to a less free drainage, 

 whence its abiUty to grow both on hill sides and on gently sloping 

 ground with less or more binding soil. A fourth class, like the 

 Pinus longifolia, requires even less moisture in the soil than the 

 preceding but the most perfect drainage, and will hence, where the 

 soil and subsoil are not freely permeable, confine itself to slopes 

 with a gradient of at least 30°. A fifth class, like some Euphorbias, 



