LEAF- ARCHITECTURE AS ILLUMINATED BY A STUDY OF PTERIDOPHYTA. 671 



The developments at the base of the leaf in T. superba go a step further than in 

 Osmunda, for the broad lateral flaps extend across the face of the petiole, so as to 

 form a continuous envelope, which ensheathes the next younger leaf. In this respect 

 it is comparable with that seen in the Marattiaceae and Ophioglossaceae.* 



A comparison of these three examples from the Osmundacese thus shows a near 

 similarity in the venation-system, but a difference in the degree of laciniation. It 

 lies near to hand to suggest that the entire cotyledon of Osmunda is a result of 

 " webbing " of a laciniate type, such as T. superba ; the vascular skeleton remaining 

 of the same fundamental type, and subject in either case to the same sympodial 

 modifications. On the other hand, the sympodial development replaces the equal 

 dichotomy relatively late in Osmunda, while in T. superba it is already advanced 

 even in the cotyledons. The diversity thus seen in the Osmundacese is widespread 

 in Ferns. The primary steps of equal dichotomy so well seen in O. regalis are apt 

 to be abbreviated, or wholly omitted. 



The modifications of the dichotomy from the equal to the sympodial thus far 

 seen have been described in plain words. They all fall under a well-known scheme, 

 designated the scorpioid sympodial dichotomy, t The differences which they show 

 depend chiefly upon (i) the degree of sympodial development, (ii) the relative 

 distance intervening between the successive forkings, and (iii) the number of 

 these forkings. 



Schizseaceas. 



Few personal observations have been made on the juvenile leaves of any 

 Schizaeaceae, excepting Aneimia adiantifolia. But that is little matter for regret, 

 since this family has been very adequately studied by Prantl. \ The genus Schizaea 

 regularly maintains the dichotomous branching in the vegetative region of the adult 

 leaf. But the matter is not so clear in the fertile region, where the branches are 

 crowded together in a dense group and folded so as to face one another. If 

 transparent preparations be made from the adult leaf, the branching may be traced 

 through the vascular connections, as seen in S. dichotoma (fig. 6 bis). In (a) the 

 leaf-tip has been halved, and the view from the inner surface shows seven pinnae, of 

 which the uppermost and lowest have themselves forked. The vascular strands all 

 gather up to the main supply in a way that is made more clear by (6), which shows 

 the adaxial aspect of another specimen. The pinnae are in apparently opposite pairs, 

 and the leaf ends in a bifurcation. The arrangement is to be interpreted thus : the 

 pinnae are not in exact pairs, but are really formed in a close sympodial series. The 



* Compare Phil. Trans., part ii, 1884, p. 577, plate xxxvii, fig. 8. 



t There have been misunderstandings in the use of terms applied to branch-systems. The use of the terms 

 here adopted is that of Hofmeister (Allgemeine Morphologie, p. 435) as applied to sympodial developments of 

 monopodial branchings. The term " scorpioid" in this sense was introduced by Bravais, while Schimper designated 

 this type of sympodium a Gicinnus. The terminology founded for monopodial systems may be adopted for designat- 

 ing the similar developments in dichotomous systems. 



\ Schizaeaceen, 1881. 



