704 PROFESSOR F. O. BOWER ON 



equal dichotomy is omitted. In this case the pinnae are all of monopodial origin, 

 and it is the phylogenetically later region that constitutes the adult leaf. A step 

 further is seen in the bud-scales of the Osmundaceae, Cycads, and Angiosperms, where 

 the effective region originates basally. 



XI. The distal branching of the leaf-primordium results in a lobed, or webbed, 

 blade. But other developments not referable to this appear in some cases at the 

 leaf-base. They are often included under the term " stipules." They may be absent 

 from whole families, but constantly present in others; and this applies both to Pterido- 

 phyta and to Flowering Plants. Among the former the Osmundacese, Marattiaceae, 

 and Ophioglossacese have them ; others have not. They appear to be primarily the 

 result of enlargement of the wings of the leaf-base, with or without a commissure. 

 The latter is seen in the Marattiaceae, in Todea and Stangeria, but not in Osmunda 

 or Cycas* They may have been of distinct phyletic origin in the different families.! 

 Such developments, which are usually styled " stipular," are basal in origin, and 

 distinct in nature from the distal branchings of the leaf. 



XII. The factor which becomes most prominent in determining the leaf-form in 

 the Higher Vascular Plants is intercalary growth. In them what is lost by arrest of 

 apical growth is largely made up by intercalary growth. This is effective in pro- 

 ducing the petiole, but it is also active in the blade. Here the marginal teeth (if 

 present) are produced early, by monopodial branching upon the phyllopodium. 

 They are carried outwards by transverse intercalary growth below their base, and 

 appear at maturity as the marginal teeth. Thus in the leaves of the Higher Plants 

 intercalary growth is prevalent, and its results disguise the similarity of the archi- 

 tectural scheme to that in the Pteridophyta. 



XIII. But the intercalary growth may intervene at various points in the phyllo- 

 podium, and thus in the mature state pinnae which were in close juxtaposition when 

 formed may be separated. Especially it may intervene between the lowest pinnae 

 and the rest. They would then appear at maturity at the base of the petiole, and 

 be liable to be ranked as " stipules," though differing in origin and nature from the 

 stipules described above. Something of this nature is foreshadowed in those basal 

 pinnae seen in the Cyatheaceae, and styled " aphlebiae." It is on developmental and 

 comparative grounds that the line will have to be drawn in each special case between 

 the two possible sources of the parts ranked as " stipules" viz. basal developments of 

 tvings, and pinnae left in a basal position by intercalary growth above them. 



The attempt has been made in the above pages to study the architecture of the 

 leaf in the Sporophyte on the broadest lines. All the three avenues of comparison 

 named at the outset have been used : viz. a comparative study of adult leaves ; a 

 similar study of juvenile leaves ; and reference to the fossil record. It will now 



* See Phil. Trans., pt. ii, 1884, pis. xxxviii-xl. 



f Goebel states pointedly that " the structures which were frequently considered to be stipules in the Ophio- 

 glossaceae are not of that nature" {Organography, Engl, ed., ii, p. 365). He gives no reason for this statement, 

 though he admits the use of the term for the similar growths of the Marattiacese. 



