>() BIOLOGICAL SOCIETY OF WASHINGTON. 



until they reached the highest state attainable by plants having 

 that structure. They even acquired the exogenous character, 

 but only in a rudimentary form. 



It would be wholly misleading to place the exogeny of these 

 plants on a par with that of the modern exogen. In the pine and 

 the oak, as every one knows, the bulk of the trunk consists of 

 what we call wood, that is, of concentric layers of thick- 

 willed vascular cells, giving to the trunk great strength and 

 resistance, and although in the great sequoias and in the 

 cork-oak the cortical portion, or bark, may attain a thick 

 ness of over a foot, still this is a relatively small portion of 

 the entire trunk, and contributes comparatively little to 

 its support. Now, if we imagine a tree in which the bark 

 constitutes the bulk of the trunk and the wood only 

 a comparatively narrow zone close to the central pith, we 

 shall have some idea of the exogenous cryptogamic forest tree 

 of the Carboniferous age. Something approaching it can be 

 seen on a small scale in the first year's growth of a modern 

 exogen, and in most herbaceous plants of that type, and we 

 have another approach to it in the trunks of living cycads. 



But when we speak of such thick bark it must not be sup 

 posed that we mean the dry corky and flaky exterior which is 

 popularly called bark. Thu, in the modern exogen, consti 

 tutes the greater part of the bark of old trees, but is really the 

 cast-off and, to a greater or less extent, dead matter pushed 

 outward by the annual growth of the bast and liber, or the 

 true live bark of the tree. For every exogen is also an en.- 

 dogen outside of the cambium layer. The bark grows by the 

 deposition of new matter to its interior. It w T as even so with 

 the exogenous cryptogam, only the endogenous or cortical por 

 tion, i. <?., the bark, then constituted the greater part of the 

 trunk, whereas it now forms only a thin zone at the periphery. 



