96 FLORA INDICA. 



with the increasing latitude till the 30th degree north, the 

 cold of winter rapidly increases (see the map of Isother- 

 mals). Hence many tropical species, genera, and even families, 

 which are sensitive to cold, are comparatively local when found 

 beyond the tropic, as most Palms, Cycas, Dipterocarpea (ex- 

 cept Vaticd), Aurantiacece, Connaraceee, Meliacece, Myrtacem, 

 RubiacecB, Ebenacea, and many more. Others are indifferent 

 to the cold of winter, provided they experience a great sum- 

 mer heat ; these advance far beyond the tropic, and lend a 

 more or less tropical aspect to the Flora even of the base of 

 the north-western Himalaya, in 33° north. Such are many 

 Leguminosm (as Bauhinia, Acacia, Erythrina, Butea, Dal- 

 bergia, Millettia), Bombax, Vatica, Nauclea, Combretaceae, 

 VerbenacecR, Lagerstrcemia, Grislea, Jasminem, and Bignonia 

 Indica. 



Passing from the forest vegetation to that of annual plants, 

 we find that an immense proportion of these are uniformly 

 distributed throughout India, and, vegetating only during 

 the hot rainy season, are neither exposed to drought nor cold. 

 Of these some of the most conspicuoiis are, besides Grami- 

 necB and Cyperacece, a vast number of small Legmninosce and 

 Scrophularinete, Sida, Corchorus, Nama, Blumea and other 

 Compositee, some Labiates (as Leucas, Anisomeles, etc.), Ama- 

 ranthaceae, Acanthacece, Convolvulacece, Ludwigia, Jussieua, 

 etc. 



Dr. Royle has well shown that this distribution of tropical 

 annuals and of perennial-rooted plants with annual stems is 

 not confined to the plains, but ascends the loftier mountain 

 valleys as far as the well-marked rainy season extends, and 

 that such plants only disappear where the accession of heat 

 and humidity is not sufficient in amount or regular enough 

 in period to stimulate their vegetative organs. Some of the 

 most remarkable of these extratropical examples of tropical 

 genera are species of Begonia, Osbeckia, Argostemma, Plec- 

 Iranthus, various Cyrtandraceee, ScitamineoR, Aracece, Com- 

 melynacea, and a few epiphytal Orchidea. 



