From the sea-level to an elevation of 12,000 feet Sikkim is covered with a dense forest, consisting of tall umbrageous trees, often with dense grass jungle, and in other places accompanied by a luxuriant undergrowth of shrubs. In the tropical zone Myrtaceæ, Leguminosæ, and tree-ferns are common, and the air is near saturation during a great part of the year. Vaccinia are found at from 5000 to 8000, and snow occasionally falls at 6000 feet. A sub-tropical vegetation penetrates far into the interior along the banks of the great rivers, and tree-ferns, rattans, plantains, and other tropical plants are found at 5000 feet, in the Ratong valley.[535]
I should conjecture that the extreme limit for the growth of the hardier species of chinchonæ, in Sikkim, will be found where their constant companions the tree-ferns and Vaccinia end, namely at 5000 feet; and that the best sites for such species as C. Calisaya and C. succirubra are about 1000 to 2000 feet lower, amidst the sub-tropical vegetation of the valleys.
Bhotan, which adjoins Sikkim on the east, is a mountainous district of much the same character. In its western part the mountain ranges are lofty and rugged, and the river-courses very deep and generally narrow. The climate is equable, and the humidity of the winter appears to increase in the part adjoining Sikkim. The steepness of the mountains, and the influence of the elevated mass of the Khassya hills to the south, make the lower slopes, which skirt the plains of Assam, drier than those more to the eastward. Deep narrow valleys carry a tropical vegetation very far into the interior of Bhotan, among lofty mountains capped with almost perpetual snow. These attract to themselves so much of the moisture of the atmosphere, that the bottoms of the valleys are comparatively dry and bare of forest. The flora resembles that of Sikkim.[536]
The Khassya hills in 25° N. lat. form an isolated mass, rising up from the plains of Assam and Silhet to a height of 6000 feet. They rise abruptly from the plains of Silhet to the south, and at 3000 feet tree vegetation ceases, and is succeeded by a bleak stony region, with a temperate flora, up to 4000 feet, where the English station of Churra Poorji is built. The table-land is here three miles long by two, to the eastward flat and stony, and to the west undulating and hilly. On the south there are rocky ridges of limestone. The southern side of the hills is exposed to the full force of the monsoon, and the rainfall is excessive, as much as 500 or 600 inches annually. Further in the interior the fall is less, and it gradually decreases until the valley of Assam is entered. This great rainfall is attributable to the abruptness of the mountains to the south, which face the Bay of Bengal, and are separated from it by 200 miles of Jheels and Sunderbunds. The heavy rains on the Khassya hills are quite local, as in Silhet the fall is only 100 inches. The plateau presents a bleak and inhospitable aspect, and there is not a tree, and scarcely a shrub to be seen, except occasional clumps of Pandanus. This desolation is caused by the furious gales of wind, and the extraordinary amount of rain which washes off the soil. The valleys are open, though with deep flanks, and the hill-tops are broad. The grassy slopes to the north are covered with clumps of shrubby vegetation, and the forests are confined to sheltered localities. Though the rainfall on the southern side is 600 inches, twenty miles inland it is reduced to 200 inches. The mean annual temperature of Churra Poorji is 66°, and in summer the thermometer rises to 88° and 90°. To the westward of the Khassyas lie the Garrows, which do not attain a greater height than 3000 to 4000 feet.[537]
The flora of the Khassya hills bears a greater resemblance to that of the hills in Southern India than to the Sikkim and Bhotan types. Genera and species forming masses of shrubby vegetation are identical with those of the Neilgherry sholas. It is probable that chinchona-plantations, especially of C. succirubra, might hereafter be formed advantageously on the northern slopes of the Khassyas, but it is evident that the best chances of success for the species growing at great altitudes, in South America, are offered in the Himalayan districts of Sikkim and Bhotan.
With a view to the establishment of chinchona-plantations in the Eastern Himalayas, plants have been forwarded by Mr. McIvor to the Botanical Gardens at Calcutta. On January 19th, 1862, there were at Calcutta 91 plants of C. succirubra, all except four supplied by Mr. McIvor; six of C. Calisaya from Java, and 133 of "grey-bark" species, of which 106 were supplied by Mr. McIvor, and twenty-seven were raised from the original South American seeds. Altogether there were 230 of the valuable species of Chinchonæ, besides fifty-nine of the worthless C. Pahudiana. It is intended to commence a chinchona plantation on the lower and outer range of Darjeeling in Sikkim at once, with a propagating-house on the model of Mr. McIvor's at Ootacamund; and afterwards to form a nursery for species growing at lower elevations on the Khassya hills.
There is another region in our Eastern dominions where suitable localities may be found for the cultivation of chinchona-plants, but it is as yet too little explored, and the difficulties of obtaining supplies, labour, and transport would be too great at present to allow of the possibility of forming plantations for some years to come. I allude to the recently formed province of Pegu. Dr. Brandis, the Conservator of Forests in Pegu, reports that it will be preferable to delay the introduction of chinchona-plants into that province, until their cultivation shall have proved successful in other parts.