684 CONCLUSION 



in Phylloglossum ; but in Lycopodium, Selaginella and Isoetes there is 

 dichotomous branching, often with unequal development of the shanks. 

 Ophioglossum also shows dichotomy of the roots. But in Equisetum and 

 in Ferns the branching is definitely monopodial, the lateral roots originating 

 apart from the apex of the main root ; a condition comparable with the 

 origin of the lateral buds in Equisetum^ or of the axillary buds in the Hymeno- 

 phyllaceae. The similarity of these conditions to what is seen in axis and 

 leaf is unmistakable. 



It is thus seen that in the axis, leaf, and root provision is made for 

 amplification of each several part by branching, and the methods of 

 branching seen in them all are essentially alike : each type of part may 

 remain unbranched, or it may dichotomise, or show monopodial branching : 

 it is also seen that dichotomous branching is prevalent in those forms which 

 comparison or palaeontological evidence shows to have been primitive. It 

 is natural that such analogies should exist between parts of the same 

 individual plant ; but there is no reason to see in them anything more 

 than parallel modes of amplification of parts which were throughout their 

 descent distinct in their origin, and in their nature. 1 



An analysis of even the most complex types of the vegetative system 

 in Vascular Plants involves only the factors thus disclosed, viz. the shoot 

 consisting of axis and leaves, with occasional emergences and hairs, and 

 the accessory roots. The apical growth of the shoot may be continued 

 indefinitely, with indefinite repetition of its several appendages ; or it may 

 itself be duplicated either by terminal or by lateral branchings, with or 

 without accessory roots. In fact, the whole vegetative system of the plant- 

 body, however complex, is built upon the simple shoot as the unit : its 

 apex, initiated in the first definition of polarity in the embryo, shows 

 continued apical growth with formation of an indefinite succession of 

 appendages : it may fork at its distal end : or new shoots may be initiated 

 below the apex: but still the whole plant-body is derived from the 

 extension or it may be the forking or repetition of that fundamental 

 unit the shoot. 



1 The fact that these structural analogies exist cannot rightly be held to show any 

 common origin of those parts, unless examples of dichotomy can be brought forward in 

 which one limb develops as one type of part, the other as another type ; or unless a gradual 

 transition from dichotomy to monopodial branching, such as is seen in the branching of the 

 leaves of Ferns, smooths over the transition from branchings which produce parts of the 

 same category to those which produce those of different category. Such direct evidence can 

 easily be found indicating a common origin of rachis and pinna in the leaves of Ferns ; but 

 it has never yet been produced in support of the views of Potonie or of Tansley as to the 

 common origin of axis and leaf, already alluded to above (pp. 628, 630). All the evidence 

 adduced by them is indirect ; though the structural analogies are interesting, they carry little 

 weight against the positive fact that in all observed cases the leaf originates normally as a 

 lateral appendage of the axis. 



