INTERNAL ANATOMY OF ORGANS. STEMS. 



527 



internodes, according to regular plans. These modifications disguise the 

 structure ; but it may be readily understood by means of diagrammatic 

 illustrations of some of the principal forms. 



Fig. 573. 



Fig. 575. 



Fig. 574. 



Fig. 573. Diagram of the arrangement of the fibro-vascular bundles in the stem of a 



Tree-fern. 

 Fig. 574. Diagram representing the arrangement of the fibro-vascular bundles in a 



Palm-stem. 

 Fig. 575. Another diagram, representing the upper (a) and lower (6) extremities of a 



Monocotyledonous trunk, with its fibrous layer, where the stem-bundles 



terminate and those of the root commence, enclosed by a cortical layer. 



The fibro-vascular bundles of Monocotyledons (fig. 576) being of the 

 definite or closed kind (p. 515), they acquire their full development in each 

 internode before the leaves to which they belong fall ; and hence the stems 

 of this class do not increase in diameter as a general rule, but have a 

 columnar character when they form woody trunks. But there are ex- 

 ceptions to this rule. 



It has just been stated that the fibro-vascular bundles terminate below, 

 near the periphery of the stem, and there form a more or less evident net- 

 work of fibres ; this network constitutes a kind of sheath round the gene- 

 ral mass of the stem, and is itself covered by a more or less developed 

 rind or cortical parenchyma (fig. 575, &). The tissue in the region of the 

 fibro-vascular network, or fibrous layer, remains in most cases in the con- 

 dition of cambium, as we see adventitious roots readily formed in this 

 situation. In Dractsna. Yucca, and some other woody Monocotyledons 

 the stem becomes increased in thickness with the age of the tree, by the 

 formation of layers of liber-like prosenchyma in this fibrous layer pushing 



