RELATIVE POSITIONS OF LATERAL MEMBERS. 



189 



appear as radial lines, indicated in Fig. 146 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 divergences are therefore all 

 equal, or the members have in this case the constant divergence f. In Fig. 147 

 the members are arranged in a quaternary whorl; on each circle or section there 

 stand in this case four similar members with the divergence J ; 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 are here 

 alternate, and all the members are arranged in eight orthostichies. If, on the other 



Fig. 148. — Diagram of a weakly plant of Euphorbia helioscopia ; c c the 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, B II the terminal 

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



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 to that of the corolla in 

 Primula; and in the primary roots oi Phaseolus, Tropceolum, Cucurbita, and other 

 Dicotyledons, superposed whorls of lateral roots not unfrequently occur. When 

 alternate whorls have only two members, they are said to be decussate, as in Fig. 144, 

 a very common arrangement with leaves. 



If it is required to represent by a horizontal projection not merely the diver- 

 gences on an axis but those on an axial system, such as a system of leaf-bearing 

 shoots, it may be done on the same principle, as is shown in Fig. 148. Each 



