BOTANY. 91 
immediately above another, and the number of leaves arranged along this 
interval of the spiral. The relation of the two is usually expressed by a 
fraction whose numerator indicates the number of turns, and the denomi- 
nator the number of leaves. ‘Thus, a phyllotaxis of 2 indicates that one 
leaf is immediately in a line below the 6th above it, or that there are 5 
leaves in the series and 2 turns of the spiral. The generating spiral may 
run from left to right, or from right to left, both being occasionally found in 
the branches and stem of a plant; when it is the same throughout the plant, 
the arrangement is homodromous ; when different in the branches from the 
stem, heterodromous. Sometimes the phenomena are such as to give rise to 
the supposition of two generating spirals at the same time, as in certain 
opposite and verticillate leaves. The final cause of this spiral arrangement is 
to enable all the leaves to have a nearly equal exposure to light, and thus 
prevent anything abnormal in the growth of the plant, this being in great 
measure determined by the leaves. 
The buds which are seen to form at different parts of the stem are of two 
kinds, leaf buds and flower buds, the former producing leaves, the latter 
flowers. The leaf buds consist of rudimentary leaves inclosing a growing 
vital point, which lengthens upwards or from the point of attachment. The 
flower buds consist of rudimentary leaves inclosing a fired or stationary vital 
point. 
The leaf bud, to which we shall here confine our attention, is, in external 
appearance, a collection of scales (rudimentary leaves) arranged one above 
the other in an imbricated manner. The centre or growing point which 
they inclose is cellular matter coated with a thin stratum of spiral vessels, 
the two answering to the pith and medullary sheath of exogens. By the 
growth of the leaf bud, branches are formed, such of the scales as are alive 
changing into leaves with the advancement of vegetation, and the evolution 
of the branch not slightly resembling the drawing out of the joints of a spy- 
glass. Leaf buds are formed in the axils of previously formed leaves; they 
may be terminal or lateral. The leaf buds may be made to grow on 
other plants than those from which they were originally developed, as in the 
process called budding. They may even detach themselves spontaneously 
from the axils of leaves, and germinate when planted in the ground. The 
tree may, in fact, be considered as an assemblage of leaf buds or phytons, 
which send out stems and leaves in one direction, and fibres in the other. 
In temperate climates the leaf buds are produced during autumn and 
winter, and protected from injury by the external scales (themselves 
rudimentary leaves), and occasionally by an additional downy or resinous 
investment. 
The manner in which the leaves are arranged in the bud is called their 
vernation, gemmation, or prefoliation. This is constant for the same species. 
The leaves may be either placed in simple apposition, or folded or rolled up in 
various modes. In the accompanying figures, a—g represent vertical sections ; 
h-n are horizontal. The dot represents the axis. 
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