46 FOEMS OF STEMS. 



buds at the extremity produce inflorescence, which thus terminates the 

 axis of the shoot, while the shoots of the succeeding year are from 

 axillary buds. When the branches of trees bearing terminal buds 

 have the axis of the shoot destroyed by wounds or by insects, then 

 the lateral leafy buds become developed, giving rise to anomalous 

 appearances seen in the Birch and other trees. 



Plants which form permanent woody stems above ground are 

 denominated trees and shrubs, while those in which the stems die 

 down to the ground are called herbs. The term tree (arbor) is ap- 

 plied to those plants which have woody stems many times exceeding 

 the height of a man, the lower part free from branches being the 

 trunk ; a small tree (arbusculus) is one not above 25 feet high ; a 

 shrub (frutex) has a stem about three times taller than a man, and 

 branches from near the base : an undershrub (mffrutex or fruticulus) 

 does not exceed the length of the arm ; while a bush (dumus) is a low 

 diminutive shrub, with numerous branches near the base. The terms 

 arborescent, fruticose, mffrutieose, and dumose, are derived from these. 



The cylindrical form of the trunk of trees is sometimes interfered 

 with by peculiarities in the production of woody tissue. In this way 

 protuberances are formed of various kinds. This is very remarkable 

 in some kinds of Bombax, and in the Bottle-tree of Australia, where 

 the whole stem appears in the form of a large flask or bottle, taper- 

 ing to each end, and swollen in the middle. So also, by interruption 

 to the growth of the root and other causes, knobby stems are formed, 

 as in the Yew (fig. 128). 



Stems have usually a round form. They are sometimes compressed 

 or flattened laterally, while at other times they are angular : bping 

 triangular, with three angles and three flat faces ; trigonous {rgiT;, 

 three, and ycavla, an angle), with three convex faces ; triquetrous 

 (triquetrum, a triangle), with three concave faces ; quadrangular, or 

 square ; quinquangular, or five-angled ; octangular, or eight-angled, 

 etc. Various terms are applied to the forms of stems, as cylindrical 

 or terete, jointed or articulated — that is, with contractions at intervals, 

 many-angled or polygonal. 



The stem has been called the ascending axis, from being developed 

 in an upward direction. It does not, however, always ascend into the 

 air ; and hence stems have been divided into aerial, or stems which 

 appear whoUy or partially above ground ; and subterranean, or those 

 which are entirely under ground. The latter are often called roots, 

 but they are distinguished by producing leaf-buds at regular intervals. 

 Underground stems are common in Monocotyledons, and it is often 

 found that the structure of Dicotyledonous underground stems, such 

 as Jerusalem artichokes, resemble in structure Monocotyledons. The 

 following are some of the more important modifications of stems : — 

 The Grown of the root is a shortened stem, often partially under ground, 



