64 OEGANOGEAPHY. 



happens that, soon after its appearance, instead of continuing to 

 take an upward direction into the air, it will grow along the 

 ground, or even bury itself beneath the surface, and thus by 

 ■withdrawing itself from the light and air it resembles, in such re- 

 spects, the root, with which organ such stems are, therefore, by the 

 common observer confounded. In these cases, however, a stem 

 is at once distinguished from a root by bearing modified leaves, 

 each of which has also the power of forming a leaf-bud in its 

 axil (that is, in the angle produced by the junction of the base of 

 the upper surface of the leaf with the stem). The presence of 

 leaves with leaf-buds in their axils is therefore the essential 

 characteristic of a stem, in contradistinction to a root in which 

 i such organs are always absent. 



All Flowermg Plants, from the mode in which their axis is 



developed in the embryo ( p. 11), must necessarily have a stem, 



although such stem may be very short. Those which have this 



\ organ clearly evident are called caulescent, while those in which 



i it is very short or inconspicuous are termed acaulescent or stem- 



\ less. In Flowerless Plants the stem is not necessarily present ; 



thus it is absent in all Thallogens, as already noticed (page 7). 



1. Internal Structure of the Stem in general. — A stem 

 in its simplest condition consists merely of parenchymatous cells, 

 with occasionally a central vertical cord of liber-cells. Examples 

 of such a stem may be seen, with few exceptions, in the Mosses 

 {figs. 8 and 9) and Liverworts. Such a structure however would 

 be unsuited to plants in which great strength is required, and we 

 accordingly find that in all plants above the Mosses the stem is 

 made up partly of parenchymatous cells, and partly of woody 

 tissue and vessels of different kinds, by which the requisite 

 strength and toughness are produced. In these stems therefore 

 we distinguish two systems as already noticed (p. 46), namely, a 

 Parenchymatous, or common cellular system, and a Fibro-vascu- 

 lar. The parenchymatous system grows in any direction accord- 

 ing to circumstances, either longitudinally, by which the stem is 

 increased in length, or horizontally, by which it is increased in 

 diameter. The fibro-vascular system only grows longitudinally, and 

 thus forms cords or bundles which are distributed vertically in the 

 midst of the parenchymatous. The parenchymatous system is 

 therefore also termed the horizontal system of the stem, while the 

 ; fibro-vnscular is likewise called the longitudinal or vertical system. 

 The diiferences which are found to exist in the internal struc- 

 ture of the stems of plants, are in a great measure owing to the 

 different ways in which the fibro-vascular system is distributed 

 in the parenchymatous. All these modifications may be, how- 

 ever, in their essential particulars, reduced to three great classes, 

 two of which are found in Flowering Plants, and one in Flower- 

 less. As illustrations of the two former we may take an Oak and 

 a Palm stem ; of the latter, that of a Tree-fern. 



