EXOGENS AND ENDOGENS. 77 



Indica), and the black Mangrove {Rhizophora mangle) are men- 

 tioned as examples of tliis singular conformation. 



a. The former originally arises with a single tnnik. From the ])rincipal 

 branches, when they have become so widely extended as to need additional sup- 

 port, long, leafless shoots are sent do^vn. When these shoots reach the earth, 

 they take root, and become new tmnks, in all respects similar to the first. The 

 branches thus supported still continue to advance, and other trunks to descend, 

 until a single tree becomes a grove or forest. There is, in Ilindostan, a tree of 

 this kind, called the Banyan, which is said by travellers to stand upon more than 

 3000 trunks, and to cover an area of 7 acres. The Mangrove tree is a native of 

 the West Indies. The new trunks of this tree are said to be formed from the 

 seeds which germinate without becoming detached from the branches, sending 

 down remarkably long, tapering radicles to the earth. 



§ 1. OF THE PHYSIOLOGICAL STRUCTURE OF THE EXOGENOUS STEM. 



194. The substance of herbaceous stems is soft and succu- 

 lent, consistmg ahnost wholly of cellular tissue, traversed longi- 

 tudinally by some few bundles (strings) of woody fibre and 

 vascular tissue, wliich diverge from the main stem into the 

 leaves. 



195. This is essentially the structure of the Jirst year's growth 

 of perennial plants also. Cellular tissue constitutes the frame- 

 work of the yearly shoots of the oak, as well as of the annual 

 pea, but in the former it becomes strengthened and consohdated 

 by the deposition of ligneous fibre in subsequent years. 



a. Plants differ in respect to the arrangemei\t of these fibres and vessels, and in 

 the mode of their increase ; on this difference is based that first grand distinction 

 of Phsenogamous plants into Exogens and Endogens, to which allusion has 

 already been made (126 — 7). 



196. The division of Exogens (outside growers) includes all 

 the trees and most of the herbaceous plants of temperate cli- 

 mates, and is so named because the additions to the diameter 

 of the stem are made externally to the part already formed. 



197. The division of Endogens (inside growers), including 

 the gTasses, and most bulbous plants of temperate regions, and 

 the palms, canes, &c. of the tropics, is named from the accre- 

 tions of the stem being made within the portions already 

 formed. 



198. In the exogenous structure, the stem consists of the i)it]i, 

 wood, and bark. 



