122 



PHYSICAL GEOGRAPHY. 



[PART I. 



in the valley of the Ganges, found the temperature of the 

 fresh milky juice of the Mudar (calotropis) to be but 72, 

 whilst the damp sand in the bed of the river where it 

 grew was from 90 to 104. 



Even in temperate climates such a phenomenon is cal- 

 culated to excite admiration ; but it is still more striking 

 to find the like effect rather increased than diminished 

 in the tropics, where one would suppose that the juices, 

 especially of a small and delicate plant, before they could 

 be cooled by evaporation, would be liable to be heated 

 by the blazing sun. 1 



A difficulty would also seem to present itself in the 

 instance of fruit, the juices of which have to undergo a 

 chemical change ; hence their circulation might be conjec- 

 tured not only to be slower, but even to be somewhat in- 

 dependent of the general circulation of the plant. Besides, 

 in the instances of fruit with hard skins, such as the pome- 

 granate, or with a tough leathery coating, like the mango, 

 the evaporation must necessarily be less than in those 

 with a soft and spongy covering. Yet all share alike in 

 the general coolness of the plant, so long as circulation 

 supplies fluid for evaporation ; but the moment this re- 

 source is cut off by the separation of the fruit from 

 the tree, the supply of moisture failing, the process of 

 refrigeration is arrested, and the charm of agreeable 

 freshness gone. 



It only remains to notice the aquatic plants, which 

 are found in greater profusion in the northern and 

 eastern provinces than in any other districts of the 

 island. This abundance is owing to the innumerable 

 tanks and neglected watercourses which cover the whole 

 surface of this once productive province, but which now 

 only harbour the alligator, or satisfy the thirst of the 

 deer and the elephant. 



1 See on this subject LINDLEY'S 

 Introduction to Botany, vol. ii. book ii. 

 ch. viii. p. 215. CARPENTER, Animal 



Physiology, ch. ix. s. 407. CABPEX- 

 TER'S Vegetable Physiology, ch. xi. 

 s. 407. Lond. 1848. 



