16 ILLUSTRATIONS OF THE BOTANY OF 
upper is nearly the limit to which the herbaceous plants of tropical genera extend. The 
third belt may be taken from this elevation up to the highest limits from which snow 
melts away on the southern face of the Himalayan Mountains. The bounds are in a 
ereat measure arbitrary to which each of these belts have been restricted, for the 
changes, both in temperature and vegetation, are so gradual, that it is impossible to 
draw any line where the peculiarities of one can be so clearly defined as not to interfere 
with those which are considered characteristic of another; and this difficulty is further 
increased by the change which is continually taking place in the climate and productions 
of similar elevations the further we penetrate into the Himalayas, for even where the 
mean temperature is the same, the range of the thermometer is greater, and the line of 
perpetual congelation higher in the internal than in the external ranges of these moun- 
tains. A further difficulty is also produced by the great difference in the vegetation of 
the northern and southern faces of the very same range or mountain, so that frequently 
a straight line running along the summit of the ridge may be seen dividing the luxuriant 
arboreous and shrubby vegetation of the northern face from the brown, barren, and 
grassy covering of the southern slope. This difference has been observed by all travel- 
lers, and may be ascribed in part to the greater depth of the soil on the northern face ; 
but chiefly, I conceive, to the less direct influence of the solar rays on this than on the 
southern side. 
The stations of Simla, Mussooree, and Lundour having been much resorted to for 
health, their climate and vegetation attentively observed, and offering an altitude of seven 
thousand five hundred feet, will afford a good illustration of the peculiarities of the 
central belt. The details of meteorological phenomena being confined to the intro- 
ductory chapter, it is sufficient here to remark, that with a range of the thermometer 
of 53°, from 27° to 80°, anda mean temperature of about 55° observed at this elevation 
in 80° of N. lat., we could not expect the existence of any plants either belonging or 
allied to tropical genera; and certainly we do not meet with them in the clear dry 
months of the autumn, spring, or summer, but only in the rainy season, when a 
moderate and uniform degree of evaporation, in a moist, mild, and equable atmosphere, 
is favourable to the growth of plants usually considered indicative of a tropical climate. 
These generally consist of only a few species of genera belonging to such tribes as have 
perennial roots and annual stems or their tuber-like stems secure in the earth from the 
cold of winter. 
The families and genera of which the species come into flower when the mild tempe- 
rature and moist atmosphere are so congenial, consist of Scitaminee, Canna, Hedychium, 
Roscoea, and Globba; Orchidea, Habenaria intermedia, pectinata, and marginata, 
“species of Platanthera, Pleione, Herminium, and Satyrium Nepalense ; Commeline, spe- 
cies of Commelina and Tradescantia ; and of Gramine a few species of Panicum, Era- 
grostis, and Andropogon ; of other tribes Begonia, Osbeckia, Drosera. Almost every rock 
is covered with species of Cyrtandracee and Platystemma Violoides, and the ground with 
Balsaminee ; of Acanthacee, a few species of Justicia and Ruellia are found; the 
Leguminose 
