MODIFICATIONS OF FORM 



171 



is shown in the vegetative shoot of Sedum pruinatum (Fig. 128). A 

 biological consequence is that with very numerous leaves each obtains 

 a maximum of exposure to light incident from above. It is also to be 

 remembered that, as the branching is axillary in the flowering plants, 

 the position of the branches themselves will follow the arrangement of 

 the leaves. This still furtlier defines the external form of the mature 

 organism. 



The different types of spiral are designated according to the angle between 

 the median planes of the successive leaves. This is called the angle of 

 divergence, and is expressed as a fraction of the complete circle. It is found 



9 



FIG. 129. 



Accurate drawing of a shoot of Rhododendron, seen from above. The successive 

 leaves are numbered, and it is seen that the ninth is covered by the first, while an 

 imaginary spiral including all their bases successively, will have encircled the stem 

 twice. That is, the angle of divergence between any two successive leaves is f . 

 (Reduced to J.) 



that in many plants, and even in whole families, certain angles are constant 

 in the mature shoot. This gives a comparative, or even a systematic value 

 to their observation. For example, in the Grasses, and in Iris, the angle 

 of divergence is J, that is, the leaves are alternately on opposite sides of the 

 stem, the third being above the first : they thus constitute two longitudinal 

 rows. In the Sedges, Veratrum, and other Monocotyledons the angle is J, 

 the fourth leaf being above the first, and their arrangement being in three 

 longitudinal rows. In the Rosaceae and many other Dicotyledons a common 

 angle of divergence is jths, and the leaves are arranged in five rows. Con- 

 sequently the sixth leaf will be directly above the first, while the imaginary 

 spiral threading their bases together will have passed twice round the axis 



