GROUP V. ANGIOSPERALE : MONOCOTYLEDONES. 



479 



in thickness by the formation of meristem in the pericycle from 

 which additional closed vascular bundles are developed (see p. 148, 



rig. in). 



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 

 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 when the plant 

 has reached a certain height 

 it may then grow cylindric- 

 ally : this is the reason why 

 in Palms, in the Maize, and 

 other similar erect stems, 

 there is a diminution in 

 thickness at the lower end. 

 Frequently, however, the 

 primary axis of the plant 

 perishes when it has given 

 rise to lateral shoots. 



The arrangement of the 

 leaves is at first alternate : 

 when the stem is well de- 



veloped this alternate ar- 

 rangement often passes over 

 into complex spiral arrange- 

 ments, as in Fritillaria and 

 in Palms, in which plants a 

 crown of leaves is conspicu- 

 ous. In the Grasses, and a 

 few other families, the phyl- 



Fio. 293. Longitudinal section of thegrain 

 of Zea Mais ( x about 6) : c pericarp ; n re- 

 mains of tbe stigma : / base of the erain ; eg 

 hard yellowish part of the endosperm ; tie 

 whiter less dense part of the endosperm ; c 

 scutellum of the embryo; an its apex; its 

 epidermis ; fc plumule ; w (below) the primary 

 root; ITS the coleorhiza: w (above) secondary 

 roots springing from the epicotyl (st). (After 

 Sachs.) 



lotaxis is permanently alter- 

 nate. A whorled arrangement of the foliage-leaves occurs but rarely. 

 The leaves commonly have a well-developed sheathing leaf-base : 

 they may be described as exstipulate. The lamina is usually 

 entire, simple in outline, often long and narrow, linear or ensiform, 

 more rarely orbicular, cordate or sagittate. Branched leaves occur 

 only in a few of the Aracese : the pinnate or palmate leaves of the 

 Palms acquire this form by the splitting of the originally entire 

 laminae, and the same is the case with the perforated leaves of 

 many Aracese (see p. 37). 



