NODE AND INTEKNODES. 71 



some ov all of them are developed, forming leafy divisions of the axis, which thus 

 becomes branched. 



b. Buds are said to be adventitious when they are neither terminal nor axillary. 

 Such buds generally result from some unnatural condition of the plant, as maim- 

 ing or disease, and may be formed in the internodes, or upon the roots (140), 

 or from the trunk, or even from the leaves, as in the Bryophyllum. 



170. A BRANCH, therefore, is a division of the axis, produced 

 by tlie dcvelopmeut of an axillary bud. 



171. A THORN, or spine, is a leafless, hardened, pointed, 

 woody process, with which some plants are armed, as if for self- 

 defence. Ex. Cratffigus, locust. 



a. The thora appears to be an abortive growth of a bud, resulting from the im- 

 perfect development of the growing point only, while ifs leafy coverings perish. 

 Some plants which naturally produce thorns become thornless by cultivation. 

 In such cases the buds are enabled, by better tillage, to produce branches instead 

 of thorns. Ex. apple, pear, gooseberry. 



b. Tiie thorn is distinguished from the prickle (43) by its woody structure, and 

 its connection with the wood of tlie stem, while the prickle, as of the rose, consists 

 of hardened cellular tissue, connected with tlie bark only. 



172. That point in the stem where the leaf, with its axillary 

 bud, is produced, is called the node, and the spaces between 

 them the internodes. 



a. In the internodes the fibres of the stem are parallel, but at the nodes this 

 order is inten-upted in consequence of some of the timer fibres being sent off later- 

 ally into the leaf-stalk, occasioning, more or less, a jointed appearance. Hence, 

 also, each internode contains fewer fibres, and is of a less diameter than those 

 below it, so that the axis gradually diminishes upwards. 



173. Since the branches arise from axillary buds, their ar- 

 rangement upon the stem will depend upon that of the leaves, 

 which, in all young plants, at least, are arranged with great 

 symmetry and order. 



174. It is a general law in the arrangement of the leaves and 

 indeed of all other appendages, that they are disposed spirally, 

 that is, in a hne which winds around the axis like the threads 

 of a screw. , 



a. But this an-angement is often so much disguised by disturbing causes that it 

 can scarcely be recognized. The most common modification of it is the circular, 

 which is readily explained. The spiral line is foiined by the union of two 

 motions, the circular and the longitudinal. The latter is produced in the grow- 

 ing plant by the advancement or lengthening of the axis. Now, if the latter be 



