406 



BOTANY 



The Stem 



The apex of the stem, especially in some aquatic forms like Hip- 

 puris, is conical, but much oftener it is flattened. No single initial 

 apical cell is to be found, but two or three initial layers of apical 

 tissue are present (Fig. 381). An evident epidermal layer covers 

 the apex, beneath which is the periblem, or primary cortex. The cen- 

 tral cylinder (plerome, stele) is not always clearly distinguishable from 

 the periblem, and the origin of the " procambium," or tissue which 

 develops into the vascular bundles, is not always perfectly certain. 



A typical herbaceous dicotyledonous stem in its simplest form 



13 



m 



FIG. 385. Tilia Americana. A, cross-section, B, longitudinal tangential section, 

 of the wood ( X 200) ; v, vessels ; t, tracheids ; m, medullary rays ; /, wood-fibres. 



shows a single layer of epidermal cells, within which lies a more or 

 less massive cortex, composed largely of chlorophyllous cells (Fig. 

 383, A). The cells immediately below the epidermis constitute 

 the principal mechanical tissue, as they do in the stems of Mono- 

 cotyledons. These mechanical elements may be either collenchyma 

 or fibrous cells. The central tissue of the stem (pith, medulla) is 

 usually composed of thin-walled parenchyma, which in such hollow 

 stems as those of the Umbelliferae, Dandelion, etc., is torn apart and 

 destroyed at an early period. 



The vascular bundles, except in a few anomalous cases (e.g. Pepe- 

 romia, Podophyllum, Nelumbo), are arranged in a circle surrounding 

 the pith, with a common endodermis derived from the innermost layer 

 of the periblem. They are formed of the united leaf-traces, which 



