GROUP IV. — rnAN'EHOGAMlA : AXGIOSPERM.E : MONOCOTYLEDONES. 5:^5 



The stem of Mono(;rtyledons is traversed loTigitudiTially (Fig. 

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

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

 In a few genera only, as Yncca 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 



cZ \ii 



V^ 



Fio. 346.— Grain of Tviiicum xwlQnxe, tte "Wbeftt. A Crcss-f ection tlirun^h the pericarp 

 and tCBta. Of these, ep is the epidermis, e the outer layers, and chX the chlorophylU 

 hiyer, of the perictirp : il remnants of the ovular integumeut, and n the outermoFt 

 tt.ickened layer of the nucellus; these together constitute the testa: al the alenron- 

 layer of the endosperm { x 210). B Median longitudinal secticn through the lower 

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

 theembrjo: the scutclium, »c ; V the lig ule j^f the sciitellum ; rs its vascular bundle; c« 

 itH layer of cylindrical epithelium: c the sheath of the plumule (colcoptile); i>r the grow- 

 ing-point of the stem; ?ip the hypocotyl ; I the cpiblast; r the radicle; cp the root-cap of 

 the ra<licle ; c\ the root-sheitth (cdlcorhiza) ; m pliice of exit of the radicle, corresjionding 

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

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



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 



su>rt 





M^t.^ r-o^<:«>/tC- 





