G. ARRANGEMENT OF LATERAL MEMBERS. 17 



2. Isobilateral Arrangement obtains when similar lateral mem- 

 bers arise on two diametrically opposite sides of the common axis, 

 and thus form two rows or orthostichies. Usually the members 

 of the two rows arise at different levels, so that they alternate 

 (Fig. 6 A}. In this case, also, it is possible to. construct a genetic 

 spiral ; such that at every half-turn it passes through the insertion 

 of a lateral member, and connects all the existing members in the 

 order of their age. Since the divergence is |, it is obviously quite 

 immaterial in which direction this spiral may be traced. Examples 

 of this alternate arrangement are given on the previous page. It is 

 only rarely that the members of the two rows stand in pairs at the 

 same level, thus forming superposed whorls of two members each ; 

 this is the case with the leaves of many Naiadacese, probably in 

 consequence of subsequent displacement. 



3. Dorsiventral Arrangement. This arrangement of lateral 

 members may be manifested in very different ways. In some cases 

 the common axis bears lateral members on one side only ; in others, 

 the common axis bears dissimilar lateral members on its different 

 sides. As examples of the former, the flowering shoots of Vetches 

 and their allies, which bear flowers on one side only, may be 

 mentioned. The stem of Marsilea is an example of the latter ; it 

 bears leaves on the dorsal surface, lateral branches on the flanks, 

 right and left, and roots on the ventral surface : this relation holds 

 good also in the case of Azolla and Pilularia. In Salvinia the 

 dorsal surface of the stem bears the foliage-leaves, the flanks the 

 branches, and the ventral surface the aquatic leaves : in Selaginella, 

 likewise, the leaves are borne on the dorsal and ventral surfaces, 

 and the branches on the flanks. In Utricularia and in the inflor- 

 escence of the Boraginacese, the branches are borne on the dorsal 

 surface, the leaves (when present) on the flanks. In the Lemnaceae 

 the branches are produced on the dorsal, the roots on the ventral, 

 side of the shoot. 



The members borne on the flanks, in these cases, are in rows, one 

 on each flank; and a similar serial arrangement can usually be 

 traced in the members borne on the dorsal and ventral surfaces. 

 Thus, in the inflorescences of the Boraginaceae, the flowers are 

 arranged in two longitudinal rows ; in those of the Vetches there 

 may be two rows (Vicia Faba, commonly), or many rows (Vicia 

 Graced). Similarly in some Ferns (Lygodium palmatum, Poly- 

 podium Heracleum] there is a single dorsal row of leaves. In 

 Azolla, Pilularia, and Marsilea, there are two dorsal rows of leaves ; 



M.B. C 



