GROUP TV. PHANEROGAM1A : ANGIOSPERMJ) : MONOCOTYLEDONS. 535 



The stem of Monocotyledons is traversed longitudinally (Fig. 

 132, p. 172) by scattered closed vascular bundles (Fig. 137) ; it 

 lias therefore no growth in thickness by the means of cambium. 

 In a few genera only, as Yucca and Dracaena, it grows subse- 

 quently in thickness by the formation of meristem in the pericycle 

 from which additional closed vascular bundles are developed (see 

 p. 205, and Fig. 154). 



The axis of the embryo in many cases continues to be the main 

 axis of the plant ; at first it is thin and weak, and since no 



FIG. 346. Grain of Triticum milgare, the Wheat. A Cross-section throngh the pericarp 

 and testa. Of these, ep is the epidermis, e the outer layers, and M the chlorophyll, 

 layer, of the pericarp: ii remnants of the ovular integument, and n the outermost 

 thickened layer of the nucellus ; these together constitute the testa : al the aleuron- 

 layer of the endosperm (x 240). B Median longitudinal section through the lower 

 part of a ripe grain, in the plane of the furrow. At the bottom of this to the left is 

 the embryo : the scutellum, sc ; I' the ligule of the scutellum ; vs its vascular bundle ; ce 

 its layer of cylindrical epithelium : c the sheath of the plumule (coleoptile); pv the grow- 

 ing-point of the stem; lip the hypocotyl; I the epiblast; r the radicle; cp the root-cap of 

 the radicle ; cl the root-sheath (coleorhiza) ; m place of exit of the radicle, corresponding 

 with the micropyle of the ovule,; p the funicle ; vp vascular bundle in the funicle ; / 

 lateral surface of the furrow (x 14). (After Strasburger.) 



secondary growth in thickness of the stem takes place, and since 

 the successive portions of the stem are thicker and more vigorous, 

 the whole stem gradually assumes the appearance of an inverted 

 cone; but whan the plant has reached a certain height it may 



