ANATOMY OF PLANTS. 205 



ler. The largest rivers in the world flow in South America, 

 in Upper India, and in Northern Asia, through forests of 

 immeasurable extent. 



323. 



We perceive in leaves a remarkable difference in point of 

 duration. Some of them, the evergreens, remain very long, 

 and fall off at least without any regularity. These are com- 

 monly of a firmer and more tough consistence, as they are 

 very small and needle-shaped, and contain a number of pecu- 

 liar, resinous, or oily juices. This characteristic leads us to 

 the conclusion, that the irritability of such leaves is not ade- 

 quate to their complete exhaustion. 



With respect again to other leaves, which have a fixed pe- 

 riodical change of budding and falling, we can find no other 

 cause of this but their more perfect irritability, which, having 

 been exposed for a length of time to stimuli, is at last ex- 

 hausted, as in all the higher organised beings the vital activi- 

 ty acts periodically. External accidents have indeed an influ- 

 ence upon these phenomena, but the weathering of the sea- 

 sons cannot be the only cause of this change, since in hot- 

 houses and green-houses, we see that tropical plants, which 

 enjoy the same heat and the same nourishment during the- 

 whole year, yet undergo this periodical change. 



VI. On (he Structure of Blossoms'. 



324. 



We refer to the distinction which we have stated above, 

 (89, 90. 101.), between the calyx, the corolla, and the nec- 

 tary, whilst we now apply ourselves to a more careful exami- 

 nation of the structure of these parts. We have already 

 stated, (185.), that the calyx has the same structure with the 

 leaves, as it springs from them. 



It is the Corolla, therefore, which must chiefly occupy us 

 at present. When it is not united with the calyx, it is distin- 

 guished by a surface, which, in general, is of an extremely 



