68 ON LEAVES. 



common form, they may be useful for purposes different 

 from those for which Nature originally designed them ; 

 but they should not on that account obtain the name of 

 the organ they but imperfectly imitate. 



Leaves are usually deciduous, that is to say, last but 

 one season ; there are but few exceptions of plants whose 

 leaves last two, three, and sometimes as long as four 

 years. Evergreens change their leaves annually, and the 

 plant remains green only because the young leaves ap- 

 pear before the old ones decay.* 



Emily. Is it not singular that the leaves of evergreens 

 should wither and fall in the spring, when the weather 

 becomes warm, the sap most abundant, and vegetation in 

 full vigor? 



Mrs. B. A leaf withers when the vessels which should 

 bring it nourishment are no longer capable of performing 

 that function. In autumn, the vessels of the petiole be- 

 come obstructed by a deposition of hard matter, which 

 disables them from transmitting sap, and, being no lon- 

 ger moistened by the passage of this fluid, they dry up 

 and wither; while the pabulum of the leaf, consisting of 

 an expansion of the cellular system, which is of a soft, 

 moist nature, preserves the leaf some little time after the 

 petiole has ceased to perform its functions. 



Caroline. Like an animal deprived of sustenance, it 

 feeds on its own fat, before it perishes. 



Mrs. B. The circulation of sap in evergreens being 

 more uniform throughout the year, the deposition of hard 



* The trees of equinoctial regions are perpetually verdant, and the 

 same leaves which have been noticed as unusually large, are equally re- 

 markable on account of their longevity, as they rarely fade till they are 

 six years old; yet these very trees, when removed to a colder region, are 

 in some cases annually stripped of their foliage. In its native country the 

 far famed Cydon, and also when cultivated in the south of Europe the 

 Quince tree is evergreen, though here, as we have abundant opportunity 

 to see, it annually parts with its leaves. On the other hand the Currant, 

 which was originally an inhabitant of the northern countries of Europe, 

 when transferred to the Island of St. Helena was soon crowned with pe- 

 rennial leaves, but it there produces less fruit than in its native country. 



363. When common organs assume an uncommon form what is said 

 of them! 364. How long do leaves last? 365. What is said of 

 evergreens'? 366. What is said of leaves in equinoctial regions; 

 and when removed to colder regions? 367. Of the Cydon and 

 Quince what is said? 368. What is said of the currant? 359, 

 When does a leaf wither! 370 . Why does not the leaf die immedi- 

 ately on the petiole's ceasing to perform its functions'? 



