34 8 BOTANY OF THE LIVING PLANT 



The second (ii), viz. distal branching, arises from the apical cone 

 above the youngest leaves and independently of them. It is not typical 

 of the Higher Flowering Plants, but it is a marked feature in the 

 Pteridophyta, and it is well seen in Lycopods and Ferns, and also 

 in the Algae (Fig. 289). It commonly results in the formation of 

 two equal shoots in place of one, by dichotomy. But various stages 

 of inequality of the forking are found which lead from equal dichotomy 

 to definite dominance of one shank over the other ; so that a mono- 

 podial branching may result, where one shank takes a definite lead 

 over the other, and actually precedes it in time. It is regarded as 

 probable that an association of the weaker shank of a distal branching 

 with the base of the next lower leaf may have been the actual source 

 of axillary branching. In this case the two types of branching are 

 not essentially different from one another, and both may be held as 

 repetitions of the original shoot. 



The third (hi), viz. adventitious budding, has already been described 

 in Chapter XIII., p. 246. Whether produced naturally or artificially 

 induced, the buds appear in abnormal position and number, and are not 

 to be held as parts of the shoot-unit but as accessory, resulting from 

 a new growth-stimulus. Adventitious roots may also be formed at 

 various points on the shoot-system, but neither are they to be held 

 as parts of it. Adventitious buds and roots may both be regarded 

 merely as repetitions of parts by establishment of a new growth- 

 stimulus at points which do not follow the usual sequence. The 

 parts produced may be held as generally homologous with those 

 which appear in the regular succession. 



The leaf of Dicotyledons varies greatly in size and form. It is 

 commonly described as consisting of Lamina and Petiole, with or 

 without stipules (Chapter V.). While this treatment may be con- 

 venient in relation to its functional activity, it does not give any 

 satisfactory morphological insight into the construction of the typical 

 leaf of a Dicotyledon. It is in reality a simple Rachis, or a modified 

 branch-system based on it. That this is so may often be clearly seen 

 from the cutting of the margins of the blade, or in its venation 

 (Figs. 260-3). The fact is, however, disguised by (1) the dorsiventral 

 structure, which leads to the branches, or pinnae, being ranged in two 

 lateral rows : (2) by the frequent condensation of the branches borne 

 by the rachis into a coherent blade : and (3) by the intercalation in 

 the course of its development of a petiole of greater or less length. In 

 order to place the morphological treatment of a Dicotyledon leaf upon 

 a rational basis the rachis should be distinguished from the pinnae and 



