84 YOUNG AND OLD LEAVES. 



VIII. 



YOUNG AND OLD LEAVES. 1 



OBSERVE a young leaf which has just raised 

 itself above the ground, or one still half hidden 

 between the cotyledons of a seedling, or lying 

 within the opening scales of a bud. The very part 

 which performs the functions peculiar to the leaf, 

 breathing out water and producing organized sub- 

 stances, is far behind in its development ; while 

 the ribs stand strongly out, the green tissue is 

 entirely immature. It is not merely that the ex- 

 tent of surface is small, but the skin of the leaf 

 is not really formed ; the outer walls of the epi- 

 dermal cells are not protected by cork, are neither 

 water-tight nor impenetrable by water-vapor. This 

 unprotected green tissue would soon become dry 

 if spread out to the sunshine and the wind. The 

 conditions are the same whether the young leaf 

 has just pushed out of the ground, or is expanding 

 from a bud, or pressing out from between the 



1 Translated from the German of Dr. A. Kerner von Marilaun. 

 " Pflanzenleben." Vol. I. p. 321. 



