Ii6 THE BIOLOGY OF FLOWERING PLANTS 



movements of the shoot axis, of the petiole, of a leaf joint or 

 pulmnus, or of the leaf blade itself. It is clear that the 

 adjustment of a stalked leaf is a much easier and more 

 accurate process than that of a sessile one. 



If we examine the erect shoots of a clump of garden 

 mint growing in the open, more or less evenly illuminated 

 from all sides, and shaded, again more or less uniformly, by 

 neighbouring shoots only, we find a simple type of arrange- 

 ment. The leaves occur in pairs, and each succeeding pair 

 stands at right angles to the one below {decussate) ; each 

 leaf is nearly horizontal. The shading of the lower leaves 

 by those above is thus largely avoided, for the rays of light, 

 falling on the plant in a slant from above, have a much larger 

 space to pass through than they would have if each leaf 

 pair stood directly above the next lower. Further, the leaves 

 decrease in size upwards, and the greater spread of the larger 

 lower leaves is not shaded by the smaller leaves above. 

 Towards the base of the stem the oldest leaves are smaller ; 

 in the grown plant they have already ceased to function, 

 and they wither away. 



A more complicated arrangement is seen in the sunflower. 

 The first leaves are again arranged in crossed pairs, but soon 

 a spiral arrangement sets in. Starting with a leaf low down 

 on the stem, and following the spiral upwards, we find that 

 we pass twice round the stem, and only when we reach the 

 sixth leaf in succession do we arrive at one standing directly 

 above the first ; this arrangement is perfectly definite, 

 and can be characterised by the fraction |. The de- 

 nominator indicates that we can treat the leaves in groups 

 of five, the first leaf of each group standing directly above 

 the first of the group below, the second above the second, 

 and so on. The numerator indicates that if we draw a 

 spiral through the leaf bases, then in each group the spiral 

 passes twice round the stem. We could define the relation 

 otherwise, namely by the angular divergence between two 

 successive leaves, which in this case, is | of 360°, or 144°. 

 This I spiral phyllotaxy is characteristic of many plants. 

 In other plants other arrangements occur, expressed by the 



