82 FLORA INDICA. 



The normal mean temperature of the equator is stated by 

 Dove to be a very little below 80°, but this is somewhat 

 exceeded in many parts of continental India. The normal 

 mean temperature scarcely diminishes at all between 0° and 

 10° N. lat. Between 10° and 20° it diminishes 2i°; between 

 20° and 30°, 7°; and between 30° and 40°, 13-3°. In 20° N. 

 lat. therefore the diminution may be estimated at about half 

 a degree of temperature, and in 30° N. lat. at 1° of tempera- 

 ture, for a degree of latitude. In India, however, the mean 

 temperature does not diminish so rapidly, owing to the in- 

 crease of the mass of land to the northward, which, as has 

 been shown, becomes excessively heated in summer. The 

 normal difference of temperature between summer and winter 

 is least at the equator, and increases with the latitude ; and 

 this effect is enhanced in India by the increase in the mass 

 of land, which makes the summers hotter and the winters 

 colder than the average. 



The phenomena of vegetation are less dependent upon the 

 mean temperature of the year than upon that of the season 

 of growth : thus, within the tropics, vegetation is active at 

 all periods of the year, but in the cooler temperate zone, and 

 at considerable elevations on the mountains of the tropics, 

 only during the summer season. It is therefore important m 

 the investigation of climate with regard to its application to 

 botany, to know the mean temperature of each of the four 

 seasons, and, if possible, that of each month. 



The only other important element by which climate is af- 

 fected, is elevation above the level of the sea. The dimi- 

 nution of temperature as we ascend (on the surface of the 



genera, and individual species are extremely sensitive to the amount of mois- 

 ture in the air, and its fluctuations. Some plant*. e\re confined to perennial hu- 

 midity, others to perennial drought, whilst still others are dependent on acces- 

 sions of heat or drought at certain fixed periods, for life and health or the 

 means of propagation. Comparatively few observations on temperature, and 

 those in certain months only, give us a sufficient approximation to the re- 

 quirements of a plant in that particular, but the hygrometrical observations 

 should be continued throughout the year. 



