68 ORGANOGBAPHY. 



donous; endogenous stems Monocotyledonous ; and acrogenous 

 stems Acotyledonous. For reasons which we shall describe here- 

 after, the latter terms are in some cases to be preferred to the 

 former. In the succeeding pages we shall use them indiscrimi- 

 nately. With these general remarks on the structure of the three 

 T^- jgQ kinds of stems we now proceed to 



"* ' describe them respectively in detail. 



A. Exogenous ob Dicotyledo- 

 nous Stem. — Plants with exogenous 

 stems are far more abundant in all 

 parts of the globe than those of 

 endogenous and acrogenous growth. 

 This kind of stem is alone found in 

 the trees and large shrubs of this 

 country, and of all the colder regions 

 of the earth. In warmer climates it 

 occurs associated with others pos- 

 sessing endogenous and acrogenous 

 Fig. 160. Horizontal section of the structure, 

 first year's shoot of an Exoge- t >.i i ^ j_ ^i 



nous or Dicotyledonous stem. In the embryo state, the exogenous 

 m. Pith, r Medullary rays. t. gtem is entirely composed of paren- 

 Spiral vessels forming the me- , -r, , "^ ^ .^ 



duiiary sheath, on the outside of chyma. But as soon as growth com- 

 which are the^other elements of niences. Some of its parenchymatous 

 the vascular bundle, b. Bark. ■,^ ■, i i •'^ i • . i 



cells become developed into vessels 



and wood-cells, so as to form the indefinite vascular bundles 

 which are characteristic of such a stem. These bundles are at 

 first separated from each other by large intervening spaces of 

 parenchyma {fig. 160), but as growth proceeds they continue 

 to enlarge, while at the same time new vascular bundles are 

 formed between them, so that they ultimately form at the end 

 of the first year's growth a zone of vessels and wood-cells round 

 the central mass of parenchyma, interrupted only at certain 

 points by projections of this parenchyma in the form of radi- 

 ating lines. This zone is also surrounded by an external layer 

 of parenchymatous tissvie which is connected with the central 

 parenchyma by the radiating lines already alluded to. The 

 stem then presents the following parts {fig. 160). 1. A central 

 mass of parenchyma, m, which is called the Medulla or Pith ; 

 2. An interrupted zone or ring of wood-cells and vessels, form- 

 ing the Wood, t ; 3. An external zone of parenchyma, or Bark., h ; 

 and 4. The radiating lines, r, connecting the pith and the bark, 

 called the Medullary rays. On the outside of the wood is the 

 cambium layer ; and the bark is also invested by the epidermis 

 already fully described. Such is the structure of all exogenous 

 stems which die annually. 



The stems of plants which live more than one year, as those 

 of trees and shrubs, at first resemble those which are herbaceous 



