18 PART I. MORPHOLOGY. [ 7 



in Selaginella there are two ventral and two dorsal rows of leaves, 

 and in Salvinia two ventral and four dorsal rows. 



The whorled arrangement is not excluded by dorsiventrality : 

 for instance, in Salvinia, the leaves are arranged in alternating 

 whorls of three, two of the leaves being borne dorsally, and the 

 third ventrally, and thus the four dorsal and the two ventral rows 

 of leaves are produced. 



The affinity between the dorsiventral arrangement and the 

 isobilateral is indicated by the fact that whilst many axes develope 

 their lateral appendages on their flanks, they eventually come to 

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

 plants produce 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 (p. 16). 



Dorsiventral or isobilateral 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 primary shoot of the seedling are arranged radially, whilst 

 on the twigs of the adult plant the leaves are arranged bilater- 

 ally (see p. 16). 



7. Development of Branch-Systems. Just as it is 

 possible to ascertain the laws governing the relative positions of 

 all members growing in acropetal succession from a study of the 

 leaves (which are alway developed in that order), so the study 

 of the branching of stems will lead to the general laws which 

 regulate branching. Any member with its branches composes a 

 branch-system ; and every branching member is, with reference to 

 its branches, the axis of a system. The following types of 

 branch-systems may be distinguished, according to the arrange- 

 ment of the members : 



1. The branching is termed a Dichotomy or Polytomy, when 

 the direct apical growth of a member ceases, two or more grow- 

 ing-points which are equally vigorous, at any rate at their first 

 development, being formed at the apex. The member which bears 

 the branches is called the base or podium, and each of these 

 branches may become the base of a new dichotomy or polytorny. 

 They may either continue to grow with equal vigour, and then, 

 in the case of a dichotomy, the branching remains distinctly 

 bifurcate (Fig. 9 A) ; as in the leaf of Schizcea dichotoma, where 



