IV] 



BUDS AND BRANCHING 



75 



a leaf at once, so placed that when mature the shank that bears it appears 

 as a small bud at the base of the strongly developed leaf The arrested 



I' 



I' 



_^^ :ate^:. fjm(-' 



Fig. 69. Young plant of Pteridinm aqitjlinitiii, bccn fioni the 

 convex side (obliquely downward directed) of the curved 

 primary axis, and showing the first dichotomy, with downward 

 directed shanks. «' = primary axis; a" = shanks of first dicho- 

 tomy; /' = leaves borne on the primary axis; /" = leaves borne 

 on shanks of dichotomy; ;-i = roots attached to the primary 

 axis; ;-^ = roots on shanks of dichotomy. 



bud may take up active growth later (Fig. 70). That this is the probable 

 evolutionary relation of the parts is indicated by the fact that the region 

 between the insertion of the bud ji ^ 



and the forking which gave rise 

 to it shows characteristic stem- 

 structure rather than that typical 

 of the leaf From this it may be 

 concluded that here it is a leaf 

 that is borne on an abbreviated 

 axis, not a bud that is seated on 

 a leaf-base. A similar interpre- 

 tation will apply in the case of 

 Stromatopteris, which has been 



anatomically described with like ^\ 7°. . Diagram constructed so as to show the 

 ^ branchmg m Pieridiutn on a hypothesis of unequal 



results by Thompson (59, p. 152). dichotomy modified from Velenovsky, Fig. 169. 



T^, I ^1 ■ ^ -y ^ r. Sections cut below a bud, as at 2.2, show stem- 



Though the pomt has not often structure, with slight bias towards leaf-characters 



on the side away from the dormant bud. Sections 

 cut above the bud show characteristic structure of 

 the petiole. Thus the anatomy supports the hypo- 

 thesis of dichotomy with arrest of one of the branches, 

 which forthwith produces a leaf. The numerals in- 

 dicate the successive branchings on this hypothesis, 

 actually on left, and diagrammatically on right. 



been thus investigated, and though 



the anatomical test may not always 



be decisive in such cases, examples 



of varying degrees of inequality 



of distal forking can easily be 



multiplied, leading to conditions where one shank is represented only by a 



dormant bud associated with the base of its own first leaf 



