LEAF-ARCHITECTURE AS ILLUMINATED BY A STUDY OF PTERIDOPHYTA. 665 



in S. verticillatum. Potonie states* that on the whole the size of the leaves 

 increases in passing from the older to the later horizons, but that the converse 

 holds for the cutting of the blade. S. tenerrimum from the lower horizons with 

 smaller leaves has quite narrow, linear, radiating segments, while S. Thoni from 

 the " Unterrothliegenden " has large undivided leaves. This is an important point 

 for comparison with the Ferns ; but in this connection it must be remembered that 

 in S. cuneifolium both linear and webbed leaves are found on the same specimen. 

 This fact does not invalidate Potonie's point, but it detracts somewhat from 

 its value. 



Filicales. 



In dealing with this, the largest group of the Pteridophyta, which shows so great 

 a diversity in foliar structure, those which are held to be relatively primitive types 

 will be taken first, and the descriptions will relate chiefly to their juvenile leaves. 

 These commonly prefigure in some measure the characters of the adult leaf. But 

 it will be seen from numerous examples that there is a greater similarity in the 

 juvenile leaves of Ferns at large than is seen in their adult leaves. In the former, 

 dichotomy, either with equal branching or sympodially developed, is in most cases 

 a quite obvious feature. 



Osmundacese. 



It is convenient first to describe the case of Osmunda regalis, as its juvenile 

 leaves show a condition which is unusually regular. But, however exceptional this 

 degree of regularity may be in other Ferns, the study of it serves to bring into 

 prominence that dichotomous structure which underlies the branching in the leaves 

 of Ferns. The cotyledon has a single vascular strand traversing its stalk, which 

 forks as it enters the lamina, and each resulting shank forks again twice (fig. 1, a). 

 The two main branches of the first dichotomy occupy the two slightly unequal and 

 not \ery clearly marked lateral lobes ; but the left-hand branch in this particular 

 leaf is slightly the stronger, and it shows signs of a second lobing. Older leaves 

 taken from different plants, compared in succession of size, show at first a very 

 striking equality of lobation and of dichotomous venation. Examples are seen in 

 fig. 1, b-d, where this equality is strictly maintained, especially in the latter. But 

 as we pass to older leaves, of larger dimensions, the equality is gradually lost, and 

 in 0. regalis the progressive steps may be more readily followed than in most Ferns. 

 Thus in fig. 1, e, while the lobing and the dichotomy of the venation are not markedly 

 different, the left lobe is larger than the right, and the acroscopic shank of the 

 venation of the left lobe has correspondingly increased. Moreover, the basal parts 

 of the alternate right and left shanks of its successive dichotomies have disposed 

 themselves in a sympodial fashion. The left-hand lobe has also become clearly 

 divided into two parts, of which the acroscopic projects the farthest, and with a 



* Lehrbuch, p. 176. 

 TRANS. ROY. SOC. EDIN., VOL. LI, PART III (NO. 21). 98 



