BOTANY. 21 



immediately above another, and tlie 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 f 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 ; Avhen it is the same throughout the plant, 

 the arrangement is homodronions ; 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 hiiih which are seen to form at different parts of the stem are of two 

 kinds, leaf buds and ßower buds, the former producing leaves, the latter 

 flowers. The leaf buds consist of rudimentary leaves inclosing a grounnsi;- 

 vital point, which lengthens upwards or from the point of attachment. The 

 ßoicer buds consist of rudimentary leaves inclosing a ß.red 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 

 groAvth 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 dowjiy or resinous 

 investment. 



The manner in which the leaves are ai*ranged in the bud is called their 

 vernation, gemination, or prrefoliatioji. 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-7i are horizontal. The dot represents the axis. 



21 



