i68 



EXTERNAL CONFORMATION OF PLANTS. 



of the members appear as radial lines, indicated in Fig. 138 by I-V. Since in 

 this case several members stand upon each median plane, they are arranged in 

 orthostichies ; and these again are so placed that they divide the circumference into 

 five equal parts. But if the members are considered in reference to their age, as 

 indicated by the figures i-ii, it is seen that the divergence between i and 2 

 is f, as also is that between 2 and 3, between 3 and 4, and so on. The diver- 

 gences are therefore all equal, or the members have in this case on the same axis 

 the constant divergence f. In Fig. 139 the members are arranged in a quaternary 

 whorl ; on each circle or section there stand in this case four similar members with 

 the divergence \ ; but the successive whorls are so placed that the median planes 

 of one whorl exactly bisect the angle of divergence of the preceding and following 

 whorls ; the whorls here alternate, and all the members are arranged in eight 

 orthostichies. If, on the other hand, two whorls stand one over the other in such 

 a manner that their members fall into the same median planes, or cover one 

 another, they are said to be superposed. Thus, for instance, the staminal whorl is 

 superposed on that of the corolla in Primula ; and in the primary roots of Pha- 

 seolus, Tropaeolum, Cucurbita, and other Dicotyledons, there not unfrequently 

 occur superposed whorls of lateral roots. When alternating whorls have only two 

 members, the position of the members is said to be decussate, as in Fig. 136, a 

 very common case with leaves. 



If it is required to represent the divergences not merely on an axis but on 

 an axial system, such as a system of leaf-forming shoots, by a horizontal projection, 



Fig. 140.— Diagram of a weakly plant of Euphorbia Jielioscopia; crthe cotyledons; /, / the first, i— lo the later foliage- 

 leaves ; numbers 6—10 form one whorl ; at ^ / in the centre is the terminal flower of the primary shoot ; R II the terminal " 

 flower of one of the five axillary shoots ; ///, ///, /// the leaves of three axillary shoots of the second order. 



it may be done on the same principle, as is shown in Fig. 140. Each system 

 of concentric circles contains the members (in this case leaves) of an axis ; the 



