LEAF-BUDS AND BRANCHES. 



93 



Pinus, , f<s, ^f, 7f T , f, occur. Thus, while it has been shown that the 

 phylloplastic ((Zst/XAov, a leaf, and -x(7T/xoV, 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. 



178. 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 iunclions with vigour. The form of the stem is also probably 

 connected with the leaf arrangement. When leaves are opposite and 

 decussate, the stems are often square, as in Labiate plants. The ordi- 

 nary 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. 



179. 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 morphological 

 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. 



Leaf-buds. 



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

 in the axil of previously-formed leaves (fig. 202 ba, ba, ba) ; or, in 

 other words, in the angle formed between the 



stem and leaf. They are hence called axillary, 

 and may be either terminal, b t, or lateral, b a. 

 In their commencement, they are cellular pro- 

 longations from the medullary rays bursting 

 through the bark. The central cellular por- 

 tion is surrounded by spiral vessels, and is 

 covered with rudimentary leaves. In the pro- 

 gress of growth, vascular bundles are formed 

 continuous with those of the stem ; and, ulti- bf[ \ 

 mately, branches are produced which hi every 

 respect resemble the axis whence the buds first 

 spnmg. The cellular portion in the centre 

 remains as pith with its medullary sheath, which 

 is closed and not continuous with that of the 

 parent stem. Thus, in the stem and branch, this sheath forms a 



-ba 



202 



Fig. 202. Upper portion of a branch of Lonicera nigra in a state of hibernation, that is to say, 

 after the fall of the leaves ; covered with leaf-buds, b t , A terminal bud. ba, b a, b a, Axillary 

 lateral buds. Below the buds, the cicatrix or scar left by the fallen leaves is seen. 



