( ir ) 



The extent to which the vegetation is clianged by elevation, will be made clear in the 

 subsequent consideration of the forests of the Karen or Martaban hills. Vegetation 

 does as a rule change with greater elevation, but not at regular intervals. The range 

 of the lower regions is larger up to about the limit where the atmospheric moisture 

 has become considerably diminished by absorption in the lower regions ; in other words, 

 up to a height, where the atmosphere has become so clear and dry, that dampness, al- 

 though still perceptible, ceases to be a powerful factor. A thousand feet of elevation, 

 therefore, in the alpine region in the tropics affects vegetation more than a difference of 

 2000 or 3000 feet in lower regions. 



With elevation is also connected the period of flowering and fruiting of plants. It is 

 well known, tliat in temperate climates the flowering takes place later in the season in the 

 proportion as we ascend to higher regions. But in tropical countries, this rule does not 

 apply to all plants, as Mr. H. Zollinger has already (Tydsohr. v. Ned. Ind.) shown to be the 

 case iu Java, for if we attentively observe the state of development of different plants, we 

 shall find some of which the flowers open later as we ascend higher ; and others, which have 

 put forth their flowers or are already fruiting, in the higher regions ; while in the lower parts 

 they are found still in bud. Thus, for instance, in tlie beginning of March I found the Rho- 

 dodmdra and Gentimiae on Nattoung at 4000 to 5000 feet elevation in bud only, while on the 

 top of the Nattoung itself they were in full flower. If we examine such plants and compare 

 those which shew a development of flower retarded by elevation, with those whose develop- 

 ment is accelerated by the same cause, we find that the former are mostly species of a more 

 tropical nature, and therefore ascending forms, wliile the latter are more temperate and there- 

 fore descending forms : thus tlie apparent anomaly is explained. 



Here, however, I suggest caution as to the correctness of the above conclusion, plausible 

 enough as it may appear at first sight. The factors which exert influence upon the phases 

 of the life of plants are so various, that witliout special study, one may easily arrive at deduc- 

 tions diametrically opposed to a true state of things. For although I conclude from the 

 nature of the plants observed, that the premature flowering in higher elevations of these 

 hills is due to their general hypsometrieal range, it is not to be forgotten that an augment- 

 ed solar radiation, which necessarily accompanies higher regions, causes (as already alluded 

 to in my remarks on light) a more rapid development of vegetation. Hence we see, for instance, 

 in the Alps, Erigeron acre, Calluna vulgaris, Parnassia palustris, Oeiitiana germanica, &c. in 

 full flower on tlie top of high hills (6000 to 7000 feet) while in the plains they are still in bud. 

 But it should not be overlooked, that in this case the plants themselves are considerably 

 reduced in size and foliage, and therefore, their vegetative organs are reduced in the same 

 ratio as the development of the reproductive organs is accelerated, or, in scientific lauguao-e, 

 the metamoiphoais is here reduced so as to allow paramorphosis in a shorter period. 



Another peculiarity due to elevation is the fact that certain shrubby epiphytical plants, 

 which in lower regions are restricted to the highest branches of trees, descend with increasing 

 elevation and become even terrestrial in the higher regions. The cause of this, however, may 

 possibly be found in the light-loving propensities of such plants. 



The growth of trees is very much impaired at higher elevations,* where the trees become 

 lower and lower, stunted, crooked and gnarled, until they become reduced to mere shrubs in 

 the alpine regions. Strange to say we find the same peculiarities of growth produced in 

 the higher regions by climate, as are seen in the plains on very poor and sterile soils, such 

 as laterite or sand. 



4. The influence of expomre can be clearly seen on stems of trees and on rocks that sur- 

 round us. Any one who has paid attention to the cryptogams which grow upon bark or stone, 

 especially lichens, will also have noticed that in open localities these are always in greatest 

 profusion on the northern sides of stems or rocks, while the opposite ones are quite or nearly 

 free of them. In the Southern hemisphere, of course, the reverse takes place. What takes 

 place in this case on a small scale, becomes a powerful factor on a large scale in hilly countries. 



But the importance of exposure is not equally great throughout all latitudes : it ^is greatest, 

 where the difference between damp and dry season, or between winter and summer is great- 

 est. Nor have the same exposures the same influence upon vegetation in ditt'erent countries. 

 While in temperate and cold climates the S. and S. W. exposures are the favourable ones, it is 

 just the reverse in Burma and other warm countries that are under the influence of the mon- 

 soons. This contrast is due to the fact that intense dry heat is as injurious to vegetation as a 

 winter with rough northern winds in temperate zones of the Northern latitudes. 



I have, for tlie sake of brevity, called all north, north-east and east exposures in Bur- 

 ma, favourable ones ; while south, south-west and west exposures are considered by me more 

 or less unfavourable to the tropicalf vegetation. But this holds good only up to about 

 7000 or 8000 feet, at which elevations the reverse gradually takes place, as in temperate zones. 



* Dr. Sendtner ascribes this solely to solar radiation. 



t Iu using the word ' tropical" i always moau " hot and damp," whereas heat and dryness would eiFeot 

 aridity. 



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