4.] 



CHAPTER I. GENERAL MORPHOLOGY. 



29 



be increased. Hence we find changes in the divergence taking 

 place jusfc in those regions in which the size of the lateral members 

 alters, for instance, at the base and at the apex of pine-cones, and 

 at the base of the inflorescences of Compositae. Furthermore, 

 subsequent alterations may be induced by growth, either of the 

 axis or of the developing lateral members. 



2. Isobilateral Arrangement. Isobilaterally arranged similar 

 lateral members arise on two diametrically opposite sides of the 

 common axis, and thus form two rows or orfchostichies. Usually 

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

 alternate (Fig. 16). In this case, also, it is 



possible to construct a genetic spiral ; and it 

 will be 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. It is obviously 

 quite immaterial in which direction this 

 spiral may be traced. Examples of alter- 

 nate arrangement are afforded by the leaves 

 of many Mosses (Fissidens) and Ferns, the 

 leaves and branches of many trees such as 

 Elm, Hornbeam, Lime, and the leaves of 

 the Grasses. 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 some 

 Algae (Pterothamnion), and of many Naiadacese, in the latter, pro- 

 bably in consequence of subsequent displacement. 



3. Dorsiventral Arrangement. This arrangement of lateral mem- 

 bers 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 men- 

 tioned, as also the thallus of Marchantia and similar Liverworts 

 which bear scales and root-hairs on the ventral surface only. 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, and in Caulerpa among Algae. Jn 

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

 the flanks the branches, and the ventral surface the aquatic leaves ; 



FIG. 16. Diagram of 

 alternating distichous (\) 

 arrangement. 



