108 LEAF-BUDS AND BKANCHES. 



found in general that, while the number 5 occurs in the phyllotaxis 

 of Dicotyledons, 3 is common in that of Monocotyledons. 



Although there is thus, in the great divisions of the vegetable 

 kingdom, a tendency to certain definite numerical arrangements, yet 

 there are many exceptions. In speaking of Palms, which are Mono- 

 cotyledonous plants, Martius states that the leaves of different species 

 exhibit the following spirals — |, f , f f , |, A, H, |^. In the species of 

 the genus Pinus, f, ^, -i^, ^, fi, occur. Thus, while it has been 

 shown that the phyUoplastic (puXXov, a leaf, and nrXaemii, formative) 

 or leaf-formative power moves in a spiral round the axis, it has been 

 found impossible to apply phyllotaxis satisfactorily to the purposes of 

 classification. 



The spiral arrangement of the leaves allows all of them to be 

 equally exposed to air and light, and thus enables them to carry on 

 their functions with vigour. The form of the stem is also probably 

 connected with the leaf-arrangement. M. Cagnat has remarked that 

 an analogy in arrangement of leaves and character of stem may be 

 traced. The leaves of juniper are in verticils of three, and the pith 

 is triangular ; the leaves of cypress being opposite, the pith presents 

 the form of a cross. When leaves are opposite and decussate, the 

 stems are often square, as in Labiate plants. The ordinary rounded 

 stem appears to be associated with a certain degree of alternation in 

 the separate leaves, or in the different pairs of leaves when they are 

 opposite. 



The study of the structure, forms, and arrangement of leaves, 

 is of great importance, when it is considered that all parts of plants 

 are to be looked upon as leaf-formations variously modified, in order 

 to serve special purposes in the economy of vegetation. The morpho- 

 logical relations of leaves, or the varied forms which they assume, will 

 be illustrated during the consideration of the organs of reproduction, 

 and of the doctrine of metamorphosis, as propounded by Goethe and 

 others. It is only by looking upon all the organs of plants in their 

 relation to the leaf as a type, that a philosophical view can be given 

 of the great plan on which they have been formed. 



Liof-huds. 



Leaf-buds contain the rudiments of branches, and are found 

 in the axil of previously-formed leaves (fig. 219 6a, ha, Id); or, 

 in other words, in the angle formed between the stem and leaf. 

 They are hence called axillary, and may be either terminal, bt, or 

 lateral, ha. They commence as cellular prolongations from the 

 medullary rays bursting through the bark. The central cellular 

 portion is surrounded by spiral vessels, and is covered with rudi- 

 mentary leaves. In the progress of growth, vascular bundles are 



