132 THK STEM. 



cessive labors of a gi-eat number of generations. The surface or the 

 recent shoots alone are alive ; and here life is superficial, all under- 

 neath consisting of the dead remains of former generations. The 

 arborescent species are not only lifeless along the central axis, but 

 are dead throughout towards the bottom : as, in a genealogical tree, 

 only the later ramifications are among the living. It is the same 

 with the vegetable, except that, as it ordinarily imbibes its nourish- 

 ment mainly from the soil through its roots, it makes a downward 

 growth also, and, by constant renewal of fresh tissues, maintains the 

 communication between the two growing extremities, the buds and 

 the rootlets. 



233. Individuality being imperfectly realized in the vegetable 

 kingdom, the question as to what in the Pha^nogamous plant best 

 answers to the animal individual is speculative, rather than practical, 

 and may be more appropriately noticed in another place. (Part II. 

 Chap. I.) 



234. Comparison of Endogenous witli Exogenous Structure. The woody 



bundles of an exogenous stem (Fig. 18G- 188) continue to grow on 

 the outer side as long as the plant lives. In woody trunks they at 

 once become wedges with the point next the pith, and growth pro- 

 ceeds indefinitely by the stratum of perj^etually renewed tissue on 

 the outer face between the wood and the bark. Each wedge is 

 separated from its neighbor on both sides by an interposed medul- 

 lary ray, and is composed of wood on the inner side, liber on the 

 outer, and cambium or forming tissue between. Now each thread 

 or bundle of endogenous wood (204) is composed of similar or anal- 

 ogous parts, sometimes irregularly intermixed, but more commonly 

 similarly disposed. That is, the section of one of these threads ex- 

 hibits woody tissue and one or two spiral vessels on its inner border, 

 answering to the proper wood, and very thick -walled elongated cells 

 on its outer border, of the same nature as the bast-cells of Exogens ; 

 and between the two is a stratum of cells of parenchyma mixed with 

 elongated and punctuated cells answering to the proper cells of the 

 inner part of the liber. The portion of each endogenous thread, 

 therefore, Avhich looks towards the centre of the trunk, answers to 

 the wood, and its outer portion to the liber or inner bark, of the ex- 

 ogenous stem ; and the parenchyma through which the threads are 

 interspersed answers to the medullary rays and pith together. The 

 main difference between the endogenous woody threads and the ex- 

 ogenous woody wedges is, that there is no cambium-layer in the 



