24 



PART I. THE MORPHOLOGY OF PLANTS. 



[4. 



This is actually true in many cases, and such whorls are said to 

 be simultaneous. There are, however, what are termed successive 

 whorls ; whorls, that is, the constituent members of which are not 

 developed simultaneously, but in a definite sequence {e.g. leaf- 

 whorls of Characeae). These must not be confounded with spuri- 

 ous whorls, which arise in this way, that members which were 

 originally developed at different levels, come, by subsequent dis- 

 placement, so to approach each other that they appear to stand 

 at the same level (e.g. the uppermost leaves on the stem of Lilium 

 bulbiferum, and the so-called whorls of branches of the Coniferae). 

 The arrangement of the lateral members upon the surface of 

 the parent member is intimately connected with the symmetry 



of the parent member, as already 

 indicated in 2. The three forms 

 of symmetry will be considered 

 separately, regard being had ex- 

 clusively to lateral members de- 

 veloped in progressive succession. 



1. Radial Arrangement. Begin- 

 ning with the whorled arrange- 

 ment, it must be noticed, in the 

 first place, that the members of a 

 whorl are all similar, and are 

 arranged symmetrically on the cir- 

 cumference of the parent member. 

 If a whorl consists, for instance, of 

 two members, they are placed exr 

 actly opposite to each other on the 

 surface of the stem, and the dis- 

 tance between them, measured from 

 the points of insertion, will amount 

 to just half the circumference of 



the stem. Similarly, if the whorl consist of three or four members, 

 the distance between any two adjacent members will be one- third 

 or one- fourth of the circumference, and so forth. The lateral 

 distance between the points of insertion of two adjacent mem- 

 bers, measured on the circumference 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 members of any whorl lie 

 opposite to the intervals between the members of the whorls above 



FIG. 11. Stem of Lamium with whorls 

 of two leaves ; 1-1, 2-2, 3-3, the succes- 

 sive whorls. 



