254 THE ANATOMY OF STEMS. 



for instance, the common Rush, these apertures are 

 perceptible in the furrows only between the stria?, the 

 elevations being apparently free from any exhaling 

 pores. In some of the Canes and Grasses, as already 

 remarked, silex is found deposited in, or rather imme- 

 diately under the epidermis. 



These kinds of stems, even when of the largest 

 diameter, display no medullary rays, that being a char- 

 acter of the dicotyledonous class of plants ; nor do 

 such appear to be necessary, owing to the extensive 

 distribution of the cellular matter throughout the sub- 

 stance of these stems. The woody bundles, howe- 

 ver, become indurated by age, and the more external 

 being enlarged by the deposition of new ligneous mat- 

 ter, they at length occasionally touch each other, and 

 form a circle of continuous wood ; but the interior 

 bundles never attain this state, and so are always suf- 

 ficient to distinguish the stem as that of a monoco- 

 tyledon. 



Stems of this class increase in length or height ; 

 but, with very few exceptions, not in diameter. The 

 stem is gradually formed by the evolution and ascen- 

 tion of the terminal leaf-bud, and by the induration of 

 the footstalks of the fallen leaves. The whole stem 

 displays the cicatrices of the successive circles of de- 

 tached leaves, and these, becoming hardened by ex- 

 posure to the air, and the ligneous bundles within them 

 being older, as they are nearer to the surface, the sub- 

 stance of the stem is necessarily softer within, and 

 harder as it approaches the circumference. Owing 

 to the mode of growth, also, which has just been des- 

 cribed, the stem is always naked, columnar, and termin- 

 ated with leaves and fruit in the form of a magnificent 

 crown, as exemplified in the Palms. The stipe, there- 

 fore, or this kind of stem, may be regarded as a fas- 

 ces of ligneous vascular rods imbedded in cellular 



