THE STEM OF VASCULAR PLANTS. 187 



that this apparent exception only results from the 

 fibrous zones of the layers being separated by very 

 narrow medullary ones. 



Section III. 

 Of the Stem of Endogens. 



Art. I. — Of the Stem in general. 



The Stems of Endogens, considered in general, have 

 the following characters in common: — 1st, They are 

 never composed of two bodies which increase in a con- 

 trary direction with regard to each other, but present a 

 single homogeneous mass ; — 2d, They never have a true 

 medullary canal, or distinct medullary rays ; — 3d, They 

 have the oldest fibres, or layers, at the circumference, 

 and the newest in the centre. It is from this last cha- 

 racter that I have given them the name by which I call 

 them, and which indicates that they increase by addi- 

 tions to the interior. These characters are less com- 

 plicated, and a little more vague, than those of Exogens ; 

 the stems of Endogens, also, present less regularity than 

 those of Exogens. In order to avoid confusion, we shall 

 be obliged to describe them separately. 



It is, without doubt, this diversity in their form which 

 has for so long a time prevented their general characters 

 from being known. We find in the writings of Grew, 

 Malpighi, and especially in the Memoire of Daubenton 

 upon the Organization of Wood — we find, I say, in these 

 authors, exact, but scattered and inconsistent observa- 

 tions, upon the differences which the stems of Endogens 



