150 APPENDAGES OF THE STEM. 



which are usually deciduous singly, or thrown off 

 together like a cap. They are seated at the base of 

 the bud, over which they are arranged like scales. In 

 some plants the hybernacula are covered with down : 

 others with a thick adhesive gum, which is an 

 additional security against frost. The sheaths of the 

 Graminece and of bulbous and tuberous stemmed 

 plants, are only modifications of this temporary defence. 

 This appendage is particularly imposing in Crocus ; 

 and on extra-tropical trees, every bud, whether promi- 

 nent or incipient, is furnished with this kind of protec- 

 tion. 



Leaves. The leaves are the most striking appen- 

 dages of plants. As they are beautifully organised 

 they must answer some important purpose in vege- 

 table economy. They have been called the lungs of 

 the plant, no insignificant term ; and as they aggre- 

 gately present an extensive surface to all atmospheric 

 influences, they must be, from this very circumstance, 

 considered as indispensable members of the system. 



That extended surface and foliaceous structure are 

 necessary to vegetative development in general, may 

 be inferred from the fact that the stems of many of 

 what are called succulent plants, and which have few r 

 or no real leaves, are flattened so as to present as large 

 a superficial plane to the air and light as possible. 



Leaves are inspiratory as well as perspiratory 

 organs. Gaseous qualities are probably emitted as 

 well as inhaled by them ; and that they both absorb 

 and perspire away aqueous matter is well known. 



