692 PROFESSOR F. O. BOWER ON . 



nerved, and of simple form. Given dichotomy of the apex, the whole complexity of 

 Hymenophyllaceous leaves can be derived from such a source, with " webbing " 

 thrown in secondarily, as an occasional but not a general factor. But the facts are 

 not yet in hand upon which a demonstration of the point can be given for the 

 Hymenophyllaceae. 



Notes on some other Leptosporangiate Ferns. 



Representatives of all those families of living Ferns which are held to be 

 relatively primitive have now been examined as to their juvenile leaves, and the 

 relation which their structure shows to the adult state. It would be superfluous to 

 pursue this inquiry exhaustively onwards through the enormous number of Lepto- 

 sporangiate Ferns which are held to be later and derivative. Moreover, the juvenile 

 leaves of these, as well as the adult, have been the subject of many observations. 

 Thus Suminsky shows for the cotyledons of Pteris serrulata a branching readily 

 referable to dichotomy with a sympodial tendency.* This is also seen in the familiar 

 figure of Adiantum capillus-veneris,~\ where the dichotomy seems quite perfect. 

 Two other examples, selected from Ferns where the mature leaf has still an open 

 venation, will be shown here in detail, the one (Cyathea) from the superficial, the 

 other (Pteridium) from the marginal series. 



The first leaf in Cyathea insignis shows, as a rule, an obvious dichotomy 

 (fig. 25, a), though the two shanks may not be exactly equal (b). It will be noticed 

 on comparing these that while in (a) the distance along each shank from the first to 

 the second dichotomy is approximately equal, in (6) it is longer in the weaker shank 

 than in the stronger. This difference is still more marked in (c), where also the 

 inequality of the branch-systems which spring from the two is greater. This leads 

 to the condition seen in (d), where the distance between the first and second 

 dichotomies is so short that the limb of the first appears almost exactly opposite 

 to the left-hand limb of the second, giving the effect as of two almost equal pinnae ; 

 while the right limb of the second dichotomy appears as a terminal lobe. This 

 corresponds exactly to what is seen in Helminthostachys and others. The same 

 development is repeated in older leaves in the distal lobe itself (e), thus leading to 

 the elaborate state of the pinnate adult leaf of Cyathea, which is thus referable to 

 a dichotomous source. 



Comparison with the juvenile leaves of Pteridium aquilinum leads to similar 

 results, though the two are widely apart in the system. The cotyledons are some- 

 times very equally dichotomous in their branching (fig. 26, a, b) ; in others slight 

 inequalities appear (c, d, e), with shortening of the distance between the first and 

 second dichotomies on the stronger shank. This is more pronounced in cases where 

 the inequality of the lobes is greater (/, g), giving the usual appearance of a ternate 

 leaf. If development of the apparently distal lobe — really one half of the second 



* Entw. d. Farnkrauter, Berlin, 1848. f Sachs' Textbook, fig. 258. 



