( 15 ) 



eluded. It Is, therefore, interesting to find, in crossing a large alluvial valley, that a great 

 number of plants disappear in these plains, which are common on the rocky or gravelly soil 

 that we have just left, but that the same plants reappear again on the other side as soon as 

 other conditions are again present. Here permeability appears to play a great part, for the 

 change of vegetation is greatest, if we pass from alluvium to compact sandstone or other im- 

 permeable strata, while the soft permeable sandstone improves, it is true, the growth of the 

 trees very much, but does not in the same degree change the botanical character of the forests. 

 I speak here chiefly of trees. 



I will here notice one of the most striking of the many examples in Bengal, that occur to 

 me, of a marked change in the character of the herbaceous and perennial vegetation. At Titalya, 

 a station on the road from Kissenguuge to Darjiling is a bungalow, which stands upon a low 

 diluvial hillock hardly fifty feet in height, formed of silioious pebbles, cemented by sand 

 and loam. This hillock is a mere speck in the surrounding alluvium, distant about 16 to 18 

 miles from the nearest diluvial formations. Along the ravine, through which runs a cart 

 road between the bungalow and the Mahanuddee river, we meet such plants as Eriophorum 

 comosum^ in abundance, Cheilanthcs farinosa, Onychium auratum, Zornia divhylla, Apocopis sp. 

 Crotalaria alUda and acicularis, Batratherum, a Fogonatum without fruits, a terrestrial leci- 

 deous lichen not yet determined, &c. These all are plants that nowhere occur in alluvial 

 soil, and are found again only on the diluvium of the Sikkim Terai, but Eriophorum is, to 

 the best of my belief, absent there also, occurring in the Khasya and Nipal hills up to the 

 North- West Himalaya. Here either the physical or chemical influence of soil is quite 

 apparent. 



Ruined pagodas, &c. in the alluvial plains of Pegu often bear plants that are not found 

 in alluvium at all, such as Sonerila, Adiantum, CheUanthes, &e. Here also the cause must 

 be- looked for in the quality of the bricks, of which the pagodas, &c. are built. 



(2.) The influence of light is probably most practically shewn, if we pitch a tent on 

 a luxuriantly growing pasture-ground, close it and let it stand there for several weeks. The 

 longer the tent stands, the greater will be the destruction of the plants that grow on the 

 spot. About six or seven weeks are sufiicient to kill all the grass. Here the deprivation of 

 the light is the cause of the death of the plants. The influence of light afiects vegetation in 

 the tropics greatly, and I have simply to point to the evergreen forests, and more especially 

 to the tropical forests on the one side, and to the mixed forests on the other ; and the effect 

 of light becomes clear in the great difl'erence of the trees and other plants in the two oases. 

 It is not necessary, therefore, to give lists of shade and light loving plants : they are 

 quickly enough recognised, if we simply compare the vegetation of evergreen and deciduous 

 forests. 



An observation of Dr. Sendtner, in his chapter on Bavarian forest-trees, is not out of 

 place here. He tells us, that light-loving trees bear as a rule winged fruits, for the reason 

 that they are compelled to grow far from each other in order to obtain the necessary degree 

 of light. His acute observation holds good also in tropical countries, and though some 

 exceptions occur, these can be explained by other contrivances with which their fruits or seeds 

 are furnished. Thus, trees like peema {Lagerstraemia) have capsules which split loculicidally, 

 and so remain on the tree that the winged seed may be dispersed by the winds. The teak- 

 tree has its light capsules enclosed in a dense woolly cover, which again is surrounded by a 

 loose bladdery sack, so light indeed, that it is only a sport for winds, &c. 



The influence of Solar radiation makes itself chieily felt in accelerating the development 

 of the reproductive organs and in shortening the cycle of vegetative life. 



(3.) The influence of elevation is tantamount to difference of climate. It is well 

 known, that in ascending a very high mountain, we pass through different regions, (called 

 sometimes also hypsometrical zones) each of which corresponds to a different zone of geogra- 

 phical latitude, except that the atmospheric pressure, the duration of days, and seasons, and 

 the degree of moisture, are not congruent. 



The Pegu Yomah is too low to show this difference in climate clearly, but the occur- 

 rence of some temperate forms, like Ileracleum, Vaccinium, etc. in the dry forests of the hills 

 is an indication of elevation. In the Martaban hills, where peaks of more than 7000 feet 

 elevation exist, the influence of elevation upon vegetation is, however, very marked. 



Indian botanists distinguish the following three chief regions, each of which can be sub- 

 divided into two sub-regious, viz. : 



I. The tropical region, up to 6 or 7000 feet elevation, divided into a tropical (up to 

 3000 feet), and a sub-tropical (up to 6 or 7000 feet). 



II. The temperate region between 7000 (in places 6000) to 12000 feet elevation, 

 similarly distinguished into a subtemperate (up to 9 10000 feet) and a temperate region 

 (from 9 or lOOUO up to 12000 feet elevation). 



III. The alpine region between 12000 and 16000 feet elevation or more, which again 

 may be divided into an alpine (between 12000 to 16000 feet) and a glacial region (above 

 16000 feet elevation). 



