NODE AND INTERNODES. 71 
some or 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 srancu, therefore, is a division of the axis, produced 
by the development 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. Cratzgus, locust. 
a. The thorn appears to be an abortive growth of a bud, resulting from the im- 
perfect development of the growing point only, while its 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. The thorn is distinguished from the prickle (43) by its woody structure, and 
its connection with the wood of the stem, while the prickle, as of the rose, consists 
of hardened cellular tissue, connected with the bark only. 
172. That point in the stem where the leaf, with its axillary 
bud, is produced, is called the nopz, 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 interrupted in consequence of some of the immer 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 azllary 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 line which winds around the axis like the threads 
of a screw. 
a. But this arrangement 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 formed 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 
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