4.] 



CHAPTER I. GENERAL MORPHOLOGY. 



25 



and below it. Thus the members of alternate whorls are exactly 

 above each other (Fig. 11). 



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

 very plainly exhibited by means of diagrams (e.g. Figs. 12-14). 

 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 

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

 higher the insertion of the member upon the stem, the nearer to 

 the centre will be the circle of the diagram upon which its 

 insertion is indicated. 



It may be perceived in the diagram Fig. 12, that when the mem- 

 bers are arranged in alternate whorls they form twice as many 

 longitudinal series on the stem as there are members in each 

 whorl, provided, of course, that the number of members in each 

 whorl is the same. The longitudinal series, which are indicated 

 in the diagram by radii, are called or- 

 thostichies. 



This particular arrangement of alter- 

 nate whorls of two members occurs very 

 frequently, arid is termed the decussate 

 arrangement. The two members of each 

 whorl are said to be opposite. 



Examples of alternating whorls are afforded 

 by the leaves of the Characeae, of Equisetum, and 

 of Hippuris. Instances of whorls of three mem- 

 bers are found in the leaves of the common 

 Juniper. Decussate leaves occur in most Caryo- 

 phyllacese, Syringa (Lilac), Lonicera (Honey- 

 suckle), Ash, Maple ; in the last named the lateral branches are also decussate. 



FIG. 12. Diagram of a shoot 

 with alternate two - leaved 

 whorls. 0, 0, 0, 0, the four or- 

 thostichies. 1, 1, 2, 2, 3, 3, etc., 

 the successive whorls. 



It is comparatively rare for equal successive whorls to be super- 

 posed ; that is, that the members of each whorl lie exactly above 

 or below those of the others, so that there are only as many ortho- 

 stichies as there are members in each whorl. This is the case, 

 however, in some flowers (see Part III., Phyllotaxi/ of the Flower). 



When the successive whorls consist of different numbers of 

 members, as on the stem of Polygonatum verticillatum, in the 

 flowers of Pomaceae, etc., complicated relations of alternation are 

 induced which need not be further considered here. 



When the similar lateral members are arranged in a scattered 

 manner it is easy to detect that, within a certain region of the 



