236 STEM EOOT. [BOOK i. 



simplex, Helleborus foetidus, &c. Perhaps this is the reason 

 why we, in many cases, find no genuine spiral vessels in the 

 developed stem, even in the medullary sheath. 



The study of the structure of stems is a boundless field for 

 research. Up to the present time no one has given a correct 

 explanation of that curious formation among the natural 

 order of Soap worts, (Sapindacese,) where in the same stem we 

 have several centres of wood-formation, only one of which 

 occupies the axis. Nor is much that can be called satisfactory 

 known of the real structure of the stem of Phytocrene, or of 

 the analogous forms frequently occurring among Bignoniads. 



While I give all possible credit to Dr. Schleideii for the 

 ingenuity and originality of these views, I cannot say that I 

 see how they affect the distinction stated to exist between 

 Exogens and Endogens, or offer any valid objection to the 

 employment of those terms. 



SECT. III. Of the Root, or Descending Axis. 



At or about the same time that the ascending axis seeks 

 the light and becomes a stem, does the opposite extremity of 

 the seed or bud bury itself in the earth and become a root, 

 with a tendency downwards so powerful, that no known force 

 is sufficient to overcome it. It is invariably an extension of 

 the longitudinal, or fibrovascular system, except when it is 

 first born in the embryo. When put forth by stems a branch 

 of fibrovascular matter is thrown off, pierces the superin- 

 cumbent cellular tissue in part, and in part pushes it before 

 it as a covering, or causes it to be generated in consequence 

 of some specific action exercised by the young root. 



As there are latent buds, so are there latent roots. This 

 is positively asserted by M. Trecul, who has paid much 

 attention to the subject, and must be evident enough to any 

 one who ever examined a branch of the common Laurel 

 (Cerasus Laurocerasus), grown in a damp and shady place, 

 and injured by any circumstance. In such a case swarms 

 of latent roots spring up, and actually force back the bark. 



Independently of its origin, the root is to be distinguished 

 from the stem by many absolute characters. In the first 



