LEAF ARRANGEMENTS 117 



fractions i, |, |, |, yV> ^^c. This regular arrangement has 

 been the subject of extensive experimental and mathe- 

 matical treatment, but no wholly satisfactory explana- 

 tion has yet been given, though it is certain that it is an 

 expression of the interplay of forces in the meristematic 

 apical region of the shoot. The works of Jost, Thompson 

 (1917) and Church (1904) should be consulted. 



It is plain that the I spiral of the sunflower is more 

 efficient than the decussate arrangement of the mint, for 

 the leaves are more evenly distributed round the stem 

 and all available space is occupied. A glance at the foliage 

 of the sunflower shows that while the leaves overlap very 

 little, there is also very little space left between adjacent 

 leaves. The petioles rise at an angle from the stem, and the 

 blades droop at an angle from the stalk ; the whole shoot 

 is a rounded cone covered with green. The rosette plants 

 show a special case of this kind of arrangement ; excellent 

 examples are afforded by the daisy, the dandelion, the prim- 

 rose, or the plantain. They may be looked on as telescoped 

 stems of the sunflower type, and they show at a glance the 

 way in which space is utilised and shading avoided. 



Further complications may be seen in a young maple 

 tree. In the erect terminal shoots the natural decussate 

 arrangement of the leaves is conspicuous. The lower 

 leaves are large with long petioles, the upper are smaller 

 with shorter petioles. Looking down on the top of such a 

 shoot, one notices the absence of mutual shading, associated 

 with complete utilisation of available space. Below the 

 apex the side branches are inclined, and are more or less 

 shaded from vertical illumination. On these the leaf 

 blades are so disposed as to receive oblique illumination ; 

 depending on their original position on the stem, the leaves 

 may simply be bent somewhat, or the petioles may be 

 more or less twisted. Finally, in horizontal branches, we 

 find the leaf blades all brought definitely into a single plane, 

 but with their origin in four rows, above, below, and to 

 both flanks, still plainly showing. The petioles of the lowest 

 leaves may be very long, and the spaces between these may 



