LEAF-ARCHITECTURE AS ILLUMINATED BY A STUDY OF PTEP.IDOPHYTA. 669 



shown in fig. 4, a, b, c. In (a) the forking is slightly unequal, the stronger being 

 to the left. There is a regular alternation of the veins forming the sympodium, 

 and the supply to the smaller right-hand lobe may be held to be one of these veins, 

 which has developed more strongly and branched more frequently to supply the 

 marginal outgrowth. On this view it would represent a partial reversion from 

 the inequality of the forking in the sympodium towards the primitive equality, such 

 as is seen in the juvenile leaves. A similar interpretation would apply to (6), but 

 there the branching of the lateral lobe is more extensive. Such conditions lead 

 easily to that in (c), where the two lobes are very equally developed, so that it is 

 difficult to say whether one or the other is the apex of the sympodium. In other 

 words, one branching of the sympodium has reverted very perfectly to the primitive 

 equal dichotomy. , 



Fig. 4, a-c. — Various states of " furcation " of the apex of the pinnule in Osmunda regalis. ( x 4. 



Seedlings of Todea barbara, kindly supplied by Messrs Hill of Edmonton, 

 have been compared, and they show an earlier departure from the equal dichotomy 

 than in O. regalis. In fig. 5, a, b, c, a number of cotyledons, or at most second 

 leaves, are shown, and they illustrate in the succession of their lettering gradual 

 steps from equal dichotomy to sympodial branching. This applies not only to the 

 venation, but also to the lobation, which is much more marked than it is in Osmunda. 

 Fig. 5, a, is an almost diagrammatic dichotomy, and the length of the shanks between 

 the first and second branchings is approximately equal. In (b) the dichotomy is 

 slightly unequal, the right lobe being the larger, but still the distance between the 

 first and second forkings is equal in the two shanks. In (c) that length is unequal, 

 the longer being that of the stronger limb, which is unusual. The more frequent 

 condition is that seen in (d), where between the first branching and the second the 

 distance on the weaker limb is greater than on the stronger. This leads to the 

 insertion of the vascular supply of the lower lobes, right and left, being nearly 

 opposite to one another. It is still more nearly the case in (e). The transition is 

 seldom so clearly seen as in these cases, and it accords with what has already been 

 noted above in Osmunda. 



