MINUTE ANATOMY OF FLOWERING PLANTS 223 



bundle is surrounded by a sheath of thick-walled lignified 

 tissue, to which it largely ow^es its tensile strength. Once 

 formed from the general formative tissue of the stem, 

 the bundle shows no further growth, no annual increase 

 of xylem and phloem. 



STRUCTURE OF STEMS 



516. On one or the other of two types the stems of 

 phanerogamous plants are constructed. In one, the w^ood 

 is made up of separate bundles, scattered here and there 

 throughout the whole diameter of the stem. In the other, 

 the wood is all collected to form a layer between a central 

 cellular part which has none in it, the pith, and an outer 

 cellular part, the bark. 



517. An Asparagus shoot and a Cornstalk for herbs, 

 and a Rattan for a woody kind, represent the first. 

 To it belong all monocotyledons. A 

 Beanstalk and the stem of any common 

 shrub or tree represent the second; and 

 to it belong all plants with dicotyledon- 

 ous or polycotyledonous embryo. The 

 first has been called, not very properly, 

 endogenous, which means inside grow- 

 ing ; the second, properly enough, exo- 

 genous, or outside growing. 



518. Endogenous stems, those of mono- 

 cotyledons, attain their greatest size and 

 most characteristic development in Palms 

 and Dragon trees. A typical endoge- 

 nous stem has no clear distinction of pith, 

 bark, and wood, concentrically arranged, 

 no silver grain, no annual layers, no bark 

 that peels off clean from the wood. 



519. Exogenous stems, those of plants 

 coming from dicotyledonous and also polycotyledonous 

 embryos, have a structure which is familiar in the wood of 

 our ordinary trees and shrubs. It is the same in an herba- 



375. 



Structure of a 

 Cornstalk, in 

 t r ans\erse 

 and longitu- 

 dinal section. 

 The dots on 

 the cross sec- 

 tion repre- 

 sent cut ends 

 of the woody 

 bundles. 



