CHAPTER XV.— THE SPERMAPHYTA. 



377 



THE MONOCOTYLEDONS. 



Plants of this sub-class seldom show in their stems any dis- 

 tinction between wood and bark ; they are without medullary 

 rays ; their fibro-vascular bundles, which are of the closed 

 collateral variety, are not arranged radially about a central pith, 

 as are the bundles in the stems of Gymnosperms and Dicotyle- 

 dons, and there is usually no cambium zone by which the stems 

 increase in thickness. (See Vegetable Histology). 



The primary root, though present in the embryo, in most 

 cases soon ceases to grow, and nutriment is absorbed from the 

 soil, chiefly from adventitious roots, which spring laterally from 

 the stem. Moreover, these roots possess no cambium zone, have 

 no medullary rays, and undergo no important secondary changes 

 as do the roots of Dicotyledons, and the central radial bundle 

 commonly has more numerous xylem and phloem rays, and the 

 endodermis which encloses it, is usually composed of thick-walled 

 cells. 



o o 



Fig. 575. — Floral diagrams of flowers of Monocotyledons. A, typical flower of Mono- 

 cotyledon ; B, diagram of flower of Iris, with the inner staminal whorl aborted : C, dia- 

 gram of flower of a Grass with the outer perianth and inner staminal whorls completely 

 aborted, and one piece each of the inner whorl of the perianth, and of the whorl of pistils, 

 aborted. 



The leaves, except those of the Arums, Yams and Smilaxes, 

 are seldom reticulate, but are mostly parallel-veined or nerved, 

 and the weaker veins are deeply buried in the mesophyll, and do 

 not stand out so prominently on the lower surface as do those 

 of Dicotyledons ; they are not often opposite or whorled, but 

 usually arranged on one of the simpler of the alternate plans, as 



