5.] CHAPTER I. GENERAL MORPHOLOGY. 31 



The affinity between the dorsiventral arrangement and the 

 isobilateral is indicated by the fact that many axes develope their 

 lateral appendages on their flanks, though they eventually come to 

 be dorsal. For instance, the creeping shoots of Butomus and other 

 plants prod uce their leaves in two lateral rows, which, however, 

 eventually undergo displacement on to the dorsal surface : again, 

 in the twigs of the Beech, the two rows of leaves approach each 

 other on the ventral surface, and the lateral branches approach 

 each other on the dorsal. 



Dorsiventral or bilateral arrangement may not uncommonly be 

 found in the same plant with radial arrangement, but in different 

 parts : thus in the Hornbeam and the Elm the leaves of the pri- 

 mary shoot of the seedling are arranged radially, whilst on the twigs 

 of the adult plant the leaves are arranged bilaterally (see p. 12). 



5. The Mutual Relations of Dissimilar Secondary 

 Members. In most plants the position of the lateral branches of 

 the shoot is closely related to the arrangement of the leaves. In 

 radial and isobilateral shoots it is the general rule that the lateral 

 buds are developed in the angle, termed the axil, made by a leaf, the 

 subtending leaf, with the portion of the stem above its insertion 

 (see Fig. 9 kn). This kind of branching is termed axillary. 



Other relations, leading to what is termed extra-axillary branch- 

 ing, may however obtain in these shoots. This may be due to 

 displacement, so that the branch springs either from the parent 

 shoot above the axil, or from the surface of the subtending leaf. 

 It may also be due to the suppression of the subtending leaf, as 

 is frequently the case in inflorescences. In some cases (e.g. Poly- 

 siphonia elongata) a branch may be developed in the place of a 

 leaf, occupying its position in the genetic spiral. In other cases, 

 as in many Mosses and Ferns, the bud is developed below the in-, 

 sertion of the corresponding leaf ; and in some Mosses, by the side 

 of it. 



Axillary branching sometimes occurs in dorsiventral shoots (e.g. 

 Naiadea?), but the lateral branches are more frequently developed 

 by the side of the leaves. Thus, in Pilularia, Marsilia, Azolla, etc., 

 where the leaves are borne dorsally and the branches on the flanks 

 of the stem, each branch is in relation with the lower edge of the 

 corresponding leaf ; and in Utricularia and the inflorescences 

 of the Boragineas, where the branches are borne dorsally and the 

 leaves laterally, each branch is in relation with the upper edge 

 of the corresponding leaf. 



