536 



PART III. THE CLASSIFICATION OF PLANTS. 



then grow cylindrically : 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 developed this alternate arrangement often passes 

 over into complex spiral arrangements, as in Fritillaria and in 

 Palms, in which plants a crown of leaves is conspicuous. In the 

 Grasses, an a few other families, the phyllotaxis is permanently 



alternate. A whorled arrange- 

 ment of the foliage -leaves 

 occurs but rarely. 



The leaves commonly have a 

 well- developed sheathing leaf- 

 base : they may be described 

 as exstipulate, although certain 

 structures, such as the axillary 

 scales (sqnamulse intravagi- 

 nales) of Naias, Elodea, Acorus, 

 etc., and the tendrils springing 

 from the petioles of Smilax, 

 have been described as stipules, 

 but without conclusive evi- 

 dence. The scales of Naias, 

 etc., seem rather to be ligular. 

 The lamina is usually entire, 

 simple in outline, often long 

 and narrow, linear or eiisi- 

 form, 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 

 Araceas (see p. 54). 



The venation of the leaves is characterized by the fact that the 

 weaker veins do not usually project on the under surface. In 

 linear leaves, and in such as are inserted by a broad base, the 

 stronger veins run almost parallel; in broader ones, e.g. Lily of 



FIG. 347. Longitudinal section of the grain 

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

 mains of the stigma ; fs base of the grain ; eg 

 hard yellowish part of the endosperm ; ew 

 whiter less dense part of the endosperm ; so 

 scutellum of the embryo ; ss its apex ; e its 

 epidermis ; fe plumule ; w (below) the primary 

 root ; ws the coleorhiza ; w (above) secondary 

 roots springing from the epicotyl (st). (After 

 Sachp.) 



