EXOGENS AND EN©OGENS. 77 



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

 tioned as examples of this singular conformation. 



a. The former originally arises with a single tnink. From the principal 

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

 port, long, leafless shoots are sent down. When these shoots reach the earth, 

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

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

 until a single tree becomes a grove or forest. There is, in Hindostan, 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 fonned from the 

 seeds which germinate without becoming detached from the branches, sending 

 down remai-kably long, tapeiing radicles to the eaiih. 



5 1. OF THE PHYSIOLOGICAL STRUCTURE OF THE EXOGENOUS STEM, 



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

 lent, consisting almost 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. 



19-3. This is essentially the structure of the Jirst year's gi-owth 

 of j^erennial 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 consolidated 

 by the deposition of ligneous fibre in subsequent years. 



a. Plants diff"er in respect to the an-angement of these tibres and vessels, and in 

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

 of Phajnogamous 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 ah-eady foraied. 



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

 the grasses, 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 

 fonned. 



19S. Li the exogenous structure, the stem consists of the pith, 

 wood, and bark. 



