1-2 



PART I. MORPHOLOGY. 



[6 



whorl, as in the case of the upper leaves of the Tiger-lily and of 

 the whorled branches of the Pines. 



The distribution of the lateral members over the surface of the 

 parent axis is either (see 4), multilateral, bilateral, or dorsi- 

 ventral. 



1. Radial Arrangement. The arrangement of the leaves on the 

 stem (phyllotaxis') is very various ; this is particularly conspicu- 

 ous in the cases where the leaves are arranged in whorls, for 

 which reason these will be first discussed. If a whorl consists, 

 for instance, of two leaves, it is obvious that they will be placed 

 exactly opposite to each other on the surface of the stem, and that 

 the distance between them, measured from the points of insertion, 

 will amount to just half the cir- 

 cumference of the stem. Similarly, 

 if the whorl consist of three leaves, 

 the distance between any two ad- 

 jacent leaves will be one-third of 

 the circumference, and so forth. 

 The lateral distance between the 

 points of insertion of two adjacent 

 leaves, measured on the circum- 

 ference of the stem, is called their 

 divergence, and it is expressed in 

 fractions of the circumference. 



Moreover, it is a rule, though 

 not without exceptions, that the 

 successive whorls alternate, so that 

 the leaves of any whorl lie opposite 

 to the intervals between the leaves 

 of the whorls above and below it. 

 Thus the leaves of alternate whorls 

 are exactly above each other (Fig. 4). 

 This arrangement, as in fact all relation of position, may be 

 very plainly exhibited by means of diagrams (e.g. Figs. 5 and 6). 

 Such a diagram consists of a ground-plan of the stem, regarded as 

 being a cone, and looked at from above ; the insertion of each leaf 

 will lie upon one of a series of concentric circles, and the higher 

 the insertion of the leaf upon the stem, the nearer to the centre 

 will be the circle of the diagram upon which its insertion is in- 

 dicated. 

 It may be perceived in the diagram Fig. 5, that when the leaves 



FIG. 4. Stem of Lamium with whorls 

 of two leaves : 1-1, 2-2, 3-3, the suces- 

 eive whorls. 



