THE STRUCTURE OF STEMS. 



215 



united into a firm membrane. This is the epidermis, 

 and, excepting the stigma, it covers all the parts of a 

 plant exposed to the air. By suitable means the epi- 

 dermis may be separated into two parts, the outer 

 of which is not cellular, and exists sometimes in the 

 lower plants when the cellular portion is wanting. 

 A great French chemist, named Fremy, has shown 

 that it is like caoutchouc, and is named the cuticle. 



EXERCISE LXVII. 

 Structure of a Woody Bundle. 



As the woody bundles of dicotyledons are essen- 

 tially alike, and as they make up the main substance 

 of the stem, we shall get the best idea of stem-struct- 

 ure by observing their composition. Fig. 386 repre- 



FIG. 386. 



sents a highly-magnified section of one of these bun- 

 dles, with its surrounding cellular tissue. Observe 

 the region marked C. It is made up of greenish 

 cells of extreme delicacy, and it is from these cells 

 that all the other parts are produced. New cells are 

 constantly forming here, and old ones are changing 



10 



