148 EXTERNAL CONFORMATION OF PLANTS. 



the attempt is made to set up sharply defined ideas for single forms, the more does one 

 become convinced that all definition, all limitation, is arbitrary, and that Nature presents 

 gradual transitions from the indistinguishable step by step to the distinct, and finally to 

 the opposite. 



Sect. 24. Various Origin of Equivalent Members ^ — (i) The different 

 members of a plant spring out of one another ; the member produced may there- 

 fore be similar (homogeneous), or dissimilar (heterogeneous) to the member which 

 produces them. In the former case the formation of new members is ordinarily 

 termed Branching. A root, for instance, branches in the production of new roots, a 

 shoot in that of new shoots, a thallonie in that of new thallomes ; in the same sense 

 the production by a leaf of lateral leaf-structures must also be considered a case 

 of branching. On the other hand the stem produces also leaves, roots, and hairs ; 

 leaves not unfrequently produce leaf-forming shoots, sometimes roots, generally 

 hairs ; leaf-forming buds may also arise from thallomes (as in Mosses), and from 

 roots. Since, therefore, members morphologically dissimilar — stem, leaf, root, tri- 

 chome — do not differ absolutely, but only in degree, the difference between Branch- 

 ing and the production of dissimilar new parts, between homogeneous and- hetero- 

 geneous growth, must be conceived not as an opposition, but only as a gradually 

 increasing differentiation of the members which grov/ out of one another. 



(2) New members may originate either by lateral budding or by dichotomy. 

 Lateral budding occurs when the producing member, after its previous increase in 

 length at the apex (the axial structure), forms outgrowths de/ow it, which, at 

 their first origin, are weaker than the portion of the axial structure which lies 

 above them. Dichotomy (rarely Polytomy), on the other hand, is caused by the 

 cessation of the previous increase in length of a member at the apex, and by two (or 

 more) new apices arising at the apical surface close to one another, which, at least at 

 first, are equally strong, and develope in diverging directions. Lateral budding 

 may form structures which are similar or dissimilar to the axial structure ; and 

 thus there arise, by lateral budding from the stem — leaves, roots, hairs, branches; 

 from the leaf — leaflets, lacinioe, lobes, hairs, sometimes leaf-forming shoots, or even 

 roots. Dichotomy, on the contrary, never produces structures which are dissimilar 

 to the producing structure ; the divisions of a root produced by dichotomy are 

 both roots, those of a leaf-forming shoot both leaf-forming shoots, those of a leaf 

 both leaf-structures ; dichotomy hence always falls under the conception of Branch- 

 ing in the above narrower sense. 



Dichotomous branching is very common among Thallophytes, especially Algae 

 and the lower Hepaticae ; among Phanerogams it occurs only exceptionally ; 

 among Vascular Cryptogams it appears to occur in Ferns {e.g. the leaves of Platy- 

 cerium alcicorne) ; but it is the only mode of branching in all shoots and roots of 

 Selaginellae, Lycopodia, and in the roots of Isoetes. (For further details of lateral 

 branching and dichotomy see the conclusion of this section and sect. 25.) 



^ Compare the literature mentioned in the previous sections, and in addition, H. von Molil, 

 Linngea, p. 487, 1837. — Trecul in Ann. des Sci. Nat. vol. VIII. p. 268, 1847. — Peter-Petershausen, 

 Beitrage zur Entwickelungsgeschichte der Biutknospen. Hameln 1869. — Braun and Magnus, Ver- 

 handlungen des Bot. Vereins der Provinz Brandenburg 187 1 (on Oaniopsis). [Warming: Recherches 

 sur la ramification des Phanerogames. Danish with French abstract. Copenhagen 1872.] 



