INTRODUCTION. 21 



plants have stems, but in some cases they are so short and concealed 

 beneath the earth, as to be called stemless. Stems do not always ascend ; 

 sometimes they trail on the ground, or burrow beneath its surface. From 

 the size and duration of the stem, plants are divided into herbs in which 

 the stem is not woody, and dies after flowering ; under shrubs, which 

 are woody plants of small size, branched from the base; shrubs, which 

 are similar, but of greater height ; and trees, which attain a still greater 

 altitude, and seldom bear branches near the ground. When stems are too 

 weak to stand erect, they are said to be decumbent, procumbent, or prostrate, 

 and when they strike root at certain intervals, they are called creeping. 

 When they cling to surrounding objects for support, they are climbers; if 

 they coil themselves around these objects, they are tiviners. When the base 

 of the stem sends off a prostrate, slender branch, which takes root and pro- 

 duces a new plant at its extremity, it is called a runner. An erect branch 

 from the underground part of the stem, capable of producing leaves, and 

 taking root, is termed a sucker. When the branch is from an aerial part of the 

 trunk, and descends and takes root, it is a stolon. A spine is a short branch 

 of a woody plant, arising from a bud ; they sometimes bear leaves. A te?i- 

 dril is often a slender, leafless branch, capable of coiling round contiguous 

 objects, so as to afford support to the stem ; sometimes it is a modification of 

 a leaf. 



The forms of the stem when it is subterranean are fully as various, but 

 may always be distinguished from roots by having regular buds, by scars in- 

 dicating the insertion of former leaves, or by scales which are the rudiments 

 of those organs. Where the stem is prostrate, lying either on, or beneath the 

 surface of the ground and giving out roots from its under surface, it is called 

 a Rhizome, as in the calamus. All the scaly and creeping roots of the 

 older botanists belong to this class. A Tuber is a much thickened portion 

 of an underground stem, furnished with eyes or latent buds, as in the potato. 

 A Cormus is a dilated subterranean base to a stem, usually round or oval, 

 and of a uniform texture, as in the Indian Turnip. A Bulb is an under- 

 ground bud, mostly furnished with fleshy scales, producing roots from its 

 base and a stem from its centre. Bulbs may be tunicated or composed of 

 concentric layers, as in the onion, or scaly, formed of scales overlaying each 

 other, as in the lily. 



The structure of stems offers great differences in plants ; it essentially con- 

 sists of cellular and woody fibres embedded in cellular tissue, the whole co- 

 vered by a skin or epidermis, but these fibres may be arranged in a dissimilar 

 manner, principally, however, in two. In one called exogenous, the plant in- 

 creases in diameter by successive additions of new layers to the circumfer- 

 ence ; in the other or endogenous by the deposit of new woody matter towards 

 the centre, within or among the older tissue. 



The stem of an Exogen is composed of three separate parts arranged 

 concentrically ; viz. bark, wood, and pith. The bark consists of an outer 

 portion, or cellular integument, composed of cellular tissue, and covered 

 by an epidermis; and an inner part or liber, in contact with the wood, 

 and formed both of cellular and fibro-vascular tissue. At certain periods, 

 a mucilaginous product is interposed between the bark and wood, which is 

 called cambium., and is supposed to be the material from which new cells and 

 vessels are formed. The wood consists of woody fibre, ducts, and vasiform, 

 tissue arranged in zones. The pith consists of soft cellular tissue, at first 

 abounding with nutritive matter, for the nourishment of the terminating buds, 

 but afterwards becoming effete and dying. It is surrounded by a narrow zone of 



