INTRODUCTORY ESSAY. 97 



A vegetation of a different nature from any of the above 

 prevails in the extratropical regions of India during the cold 

 months only ; and, though contrasting in character with that 

 of tropical annuals, is dependent upon analogous modifications 

 of climate for its presence. This consists of annual plants 

 of the north temperate zone that do not appear within the 

 tropics (except at a considerable elevation), and which owe 

 their southward extension into India to the winter's cold, just 

 as the summer annuals owe their northward extension to the 

 heat. These flower when the tropical plants are torpid : they 

 are very numerous, comprising many European and cosmo- 

 politan genera, and even species. Besides the winter crops of 

 the Gangetic plain, consisting of Wheat, Barley, and more 

 rarely Oats, with various kinds of pulse, there are, of wild 

 plants, Ranunculus sceleratus and muricatus, Capsella Bursa- 

 pastoris, Silene conica, Alsine media, Arenaria serpyllifolia, 

 Euphorbia Helioscopia, Medicago lupulina and denticulata, La- 

 thyrus Aphaca, Gnaphalia, Xanthium, Veronica agrestis and 

 Anagallis, Heliotropium Europaeum, various Polygona, Juncus 

 bufonius, Butomus umbellatus, Alisma Plantago, and very many 

 Cyperacea, Grasses, and such aquatics as Myriophyllum, Po- 

 tamogeton nutans and crispus, Vallisneria, Zannichellia, Ra- 

 nunculus aquatilis, Lemna, and many others. 



The transition from the tropical to the temperate Flora 

 is more rapid in ascending above the level of the plains, 

 than in advancing northward at the same level; the change 

 of vegetation in a few thousand feet of ascent being much 

 greater than in as many degrees of latitude as would com- 

 pensate for the decrease of temperature experienced in that 

 ascent. In the perennially humid provinces of India the 

 climate of the base of the mountains is even more equable 

 than that of the adjacent plains, from the atmosphere being 

 more loaded with moisture. Hence in these regions a warm 

 temperate Flora (neither strictly temperate nor markedly tro- 

 pical) commences at elevations of 3-3000 feet, and prevails 

 over the purely tropical, which appears in scattered trees, 



