FOItMS OF THE LEAF-STEMS. 77 



the tree forms. In the one, named by Lindley the EXCURRENT, 

 the trunk, from the superior vigor of its terminal bud, takes pre- 

 cedence of the branches, and runs through to the summit, as in 

 the Beech, Birch, Oak, and especially in the Spruce trees with 

 oval or pyramidal forms. But in the other, the SOLVENT AXIS, 

 as seen in the Elm and Apple-tree, the trunk suddenly divides 

 into several subequal branches, which thence depart with differ- 

 ent degrees of divergency, giving the urn form to the Elm, the 

 rounded form to the Apple-tree, the depressed form to the Sloe- 

 tree (Viburnum) and Dogwood. 



227. CATJDEX is a terra now applied to the peculiar trunk of the Palms and Tree-ferns, 

 simple, branchless columns, or rarely dividing in advanced age. It is produced by the 

 growth of the terminal bud alone, and its sides are marked by the scars of the fallen leaf 

 Btalks of former years, or are yet covered by their persistent bases. The stock or cavdex 

 of the cactus tribe is extraordinary in form and substance. It is often jointed, prismatic, 

 branched, always greenish, fleshy, and full of a watery juice. Instead of leaves, its lateral 

 buds develop spines only, the stem itself performing the functions of leaves. These 

 plants abound in the warm regions of tropical America, and aflbrd a cooling, acid bev- 

 erage to the thirsty traveller when springs dry up under the torrid sun. 



228. THE VINE is either herbaceous or woody. It is a stem 

 too slender and weak to stand erect, but trails along the ground, 

 or any convenient support. Sometimes, by means of special 

 organs for this purpose, called tendrils, it ascends trees and other 

 objects to a great height ; as the Grape, Gourd, and other 

 climbing vines. 



229. The twining vine having also a length greatly disproportion ed to its diameter, 

 supports itself on other plants or objects by entwining itself around them, being destitute 

 of tendrils. Thus the Hop ascends into the air by foreign aid, and it is a curious fact 

 that the direction of its winding is always the same, viz., with the sun, from left to right ; 

 nor can any artificial training induce it to reverse its course. This is a general law 

 among twining stems. Every individual plant of the same species revolves in the same 

 direction, although opposite directions may characterize different species. Thus the 

 Morning-Glory revolves always against the sun. 



Review. 222. Carefully repeat the definition. When are they herbaceous ? When 

 woody? 223. Define Caulis, and two derived terms. 224. What is a Culm? 225. What 

 is the Trunk? Various forms of it. 226. Its characteristic difference in Beech and Elm. 

 227. Characterize the trunk of the Palm-tree Of Cactus. 228. Describe the Vine, and 

 two varieties. Are the climbers or twiners armed with tendrils? 229. The law of the 

 course of the twiners. Course of Hop Of Morning-Glory. 



