THE STEM OF VASCULAR PLANTS. 173 



itself found to be a zone of cellular tissue), is developed 

 in its turn ; either because it is no longer restrained in 

 its growth, or because it is exposed to the air and light. 

 It tends then to form again a new cellular envelope, 

 which being of the same texture as the first, and being 

 exposed to the same causes of alteration, lasts the same 

 period, and is destroyed in like manner : in fact, all trees 

 which lose their cellular envelope, lose it periodically., 



There are some plants, the stems of which present 

 very decided angles ; and which, when cut transversely, 

 present the section of the woody body perceptibly cir- 

 cular. The angular form is owing, in this case, to the 

 development or peculiar form of the cellular envelope ; 

 but as the woody body enlarges and distends this en- 

 velope, the angles are effaced and the stem becomes 

 cylindrical. It is thus that a great number of Dicotyle- 

 dons, such as the Labiatee, with square stems, &c. &c. 

 present branches of various forms, which are all trans- 

 formed, by degrees, into cylindrical stems. I know that 

 this explanation is not appHcable to all angular branches, 

 and especially compressed ones ; but it is true with re- 

 gard to several. 



The external surface of the cellular envelope being 

 exposed to the action of the air and light, changes into 

 epidermis, and presents all the phenomena which we have 

 mentioned in speaking of that organ ; but it never forms 

 a true cuticle. 



Art. III. — Of the Formation of the Branches 

 IN Exogenous Stems. 



The formation of the Branches in Exosrenous Stems 

 is sufficiently easy to understand, after the facts which we 

 have before described. 



