INTERNAL STRUCTURE. 



69 



in certain aquatic herbs), it is reduced to a few threads or 

 vessels, generally delicate, and sometimes obscure. The ac- 

 companying anatomical illustrations (Fig. 124", 125, with their 

 explanations) will give a general idea of the nature of the ana- 

 tomical elements of the stem. 



K>2. In the forming state, the whole stem is parenclryma ; but 

 an early differentiation takes place, converting certain portions 

 into woody or fibro-vascular tissue. This is arranged in two 

 ways, giving rise to two kinds of stem in phaenogamous plants, 

 which have been termed the Endogenous and the Exogenous, 1 

 meaning inside and outside growers. 



133. The two plans of stem are usually manifested in external 

 conformation as well as internal structure, and are correlated 

 with important differences in embryo, foliage, and flower. 2 Palms, 

 Lilies, Rushes, and Grasses are examples of the endogenous 

 class ; the ordinary trees and shrubs, especially those of cool 

 climates, and a large part of the herbs, are of the exogenous class. 

 In an exogenous stem, the wood occupies annual concentric layers, 

 one of each }-ear's growth ; the centre is occupied by a pith, 

 composed of parenchyma only, the circumference by a separable 

 bark ; so that a cross-section presents a 

 series of rings or circles of wood, or in 

 the first year one ring, surrounding the 

 pith and surrounded by the bark. An 

 endogenous stem has the wood in distinct 

 threads or fibro-vascular bundles, travers- 

 ing the cellular system or parenchyma with 

 little or no obvious order, and presenting 

 on the cross-section the divided ends of these bundles in the 

 form of dots ; these usually (but not always) diffused over 



1 Terms introduced by DeCandoIle, following the ideas of Desfontaines, 

 and which have played an important part in structural and systematic 

 botany ever since DeCandoIle adopted these names as those of the two 

 primary divisions of phasnogamous plants, Exogence and Endogencc. But it 

 has long been seen that the name of the second kind is not appropriate ; and 

 the older and better (though longer) names of Jussieu, Monocotyledones and 

 Dicotyledones, are reverted to. Yet the Candollean names are still much 

 employed, with due explanation, to designate the two kinds of structure of 

 the stem. 



1 Yet with some more or less valid exceptions, as when the annual stem 

 of Podophyllum and the rhizoma of Nymphsca, among dicotyledonous plants, 

 imitate the endogenous structure ; or where the pith of an evidently exogenous 

 stem, as in the Piperaceaj, has scattered woody bundles in an endogenous 

 fashion ; or where monocot3 r ledonous plants have all their woody bundles in 

 a definite circle, as in Luzula, Croomia, &c. 



FIG. 126. Section of a small Palm-stem, in two directions. 



