70 PLANT MORPHOLOGY 



slender a ground as similarity of method of branching represented in 

 them both. 



For sympodial development of a dichotomous system (and this is all 

 that such "overtopping" actually is) has occurred in cases where it 

 cannot be held to have resulted in a branching which is foliar: and 

 of this instances can be found without going so far afield as the 

 Fucacece. It has been shown by Bruchmann 1 that the first branch- 

 ing of Sdaginella spinulosa is dichotomous, and that this is probably 

 so for all species of the genus. This mode of branching may be re- 

 peated, but the later branches may lead by most gradual transitions 

 to the monopodial type. Yet no one would hold that the shoots thus 

 laterally placed are consequently foliar in their nature. Again, 

 in Lycopodium the branches of successive dichotomies often develop 

 unequally: a conspicuous example is seen in L. unilaterale, R. Br., 

 where one limb develops as a strobilus, and is pushed to one side 

 by its stronger vegetative brother. A similar unequal development of 

 dichotomous branches probably leads to such dendroid forms as 

 L. cernuum. Progressions from the dichotomous to the monopodial 

 branching are also to be seen in the case of the roots of Selaginella. 

 Such examples show that in pteridophytes progressions are found 

 from the regular dichotomous branching to its sympodial, or even 

 its monopodial development, in cases where it is impossible to rank 

 as leaves those parts which are forced to assume the lateral position. 

 This shows that such progressions are a widespread phenomenon, 

 occurring in parts of various category. If this be so, then little value 

 need be attached to the comparison of such branchings in plants 

 not nearly allied to one another; these may be held to be quite 

 distinct examples of a general phenomenon, without the one being 

 in any sense the prototype of the other. Such reflections as these 

 indicate that the comparison in mode of branching bet ween the leaves 

 of ferns and the thallus of fucoids, which forms the groundwork of 

 the view of Potonie" (or between ferns and the thalloid liverworts, as 

 may be preferred by others), are not to be held as more than distant 

 analogies; consequently they are no demonstration of the origin of 

 the leaf by a process of "overtopping." 



There remains the third view, which, however, is no new one: for 

 there have not been wanting those who have assigned a more prom- 

 inent place to the axis in the initial differentiation of the shoot. 

 Perhaps the most explicit statement on this point is that by Alex- 

 ander Braun, who remarks in his Rejuvenescence in Nature (Eng. 

 ed. p. 107), referring to phytonic theories, that "all these attempts 

 to compose the plant of leaves are wrecked upon the fact of the 

 existence of the stem as an original, independent, and connected 

 structure, the more or less distinct articulation of which certainly 

 1 Untcrs. u. Selaginella spinulosa, Gotha, 1897, p. 18, etc. 



