34 FORMS OP STEMS. 



are developed more in diameter than in height, and present a peculiar 

 shortened and thickened aspect, as Testudinaria, or Tortoise-plant, and 

 Cyclamen. 



67. Stems have a provision for a symmetrical arrangement of leaves 

 and branches ; nodes (nodus, a knot), or points whence leaf-buds are 

 produced, being placed at regular intervals. No such provision occurs 

 in roots which ramify irregularly, according to the nature of the soil. 

 The intervals between nodes are called internodes. The mode in which 

 branches come off from the nodes, gives rise to various forms of trees, 

 such as pyramidal, spreading, or weeping ; the angles formed with 

 the stem being more or less acute or oblique. In the Italian Poplar 

 and Cypress the branches are erect, forming acute angles with the 

 upper part of the stem ; in the Oak and Cedar they are spreading or 

 patent, forming nearly a right angle ; in the weeping Ash and Elm 

 they come off at an oblique angle ; while in the weeping Willow and 

 Birch, they are pendulous from their flexibility. The comparative 

 length of the upper and under branches, also gives rise to differences 

 in the contour of trees, as seen in the conical form of Spruce, and the 

 umbrella-like form of the Italian Pine (Pinus Pinea). 



68. Plants which form permanent Avoody stems above ground, are 

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

 down to the ground, and are not persistent, are called herbs. The 

 term tree (arbor) is applied 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 shrub (frutex) has a stem about three 

 times taller than a man, and branches from near the base: an under- 

 shrub (suffrutex) 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, suffruticose, and dumose, are 

 derived from these. 



69. Stems have usually a round form; but they are sometimes com- 

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

 being triangular, with three angles and three flat faces; trigonous 

 (r^iif, three, and yavlcc, 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, &c. 



70. The stem has been called the ascending axis, from being devel- 

 oped in an upward direction. It does not, however, ahvays ascend 

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

 which appear wholly 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. 

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

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

 which remains in some plants after the leaves, branches, and flower- 



