702 PROFESSOR F. O. BOWER ON 



Cruciferae that it is so. The venation in such questions will have to be examined 

 more carefully than hitherto. What emerges from examination of the illustrations 

 in Lubbock's book On Seedlings is, that the prevailing venation in the cotyledons of 

 dicotyledonous plants is of a type similar to that of an entire juvenile leaf of the 

 Ophioglossaceae or Marattiaceae ; but this subject will have to be gone over more 

 critically before comparisons can be applied in detail. 



The earliest juvenile leaves do not always show an equal dichotomy. The series 

 may be more or less abbreviated, and the construction typical of the primitive forms 

 may be omitted. This is particularly obvious in Botrychium (fig. 12) and Helmin- 

 thostachys (fig. 12). The Marattiaceae are interesting in this connection ; for while 

 Dansea (fig. 10) may show an initially equal dichotomy, in Angiopteris* the 

 cotyledon itself has distinctly sympodial development, the primitive equal dichotomy 

 being omitted. This seems to be the prevailing type in the dicotyledons. Thus 

 there is in the juveyiile leaves of Vascular Plants an orderly progression of structure 

 more or less fully indicated, hut apt to he abbreviated in certain types. That pro- 

 gressioyi may he held to he prohahly phyletic in relatively primitive types. It 

 proceeds from equal dichotomy to sympodial construction. 



VI. Where sympodial development of dichotomy is seen it may take various 

 forms. Where the favoured shank is alternately right and left, which is the 

 commonest state, the sympodium is scorpioid, as in figs. 11, 14, 15. Where it is 

 repeatedly on the same side the sympodium is helicoid, as in Plate-figs. A-F. But 

 in leaves there are two possible types of this last, in relation to the leaf as a whole. 

 The favoured shanks may be on the side towards the leaf-base, and the result may be 

 designated a katadromic helicoid sympodium, as in Matonia, Plate-fig. A. Or it 

 may be on the side towards the apex, when it is called . an anadromic scorpioid 

 sympodium, as in Pteris semipinnata, Plate-fig. E. The various types of sympodial 

 development are variously distributed in the construction of leaves, and produce 

 very characteristic effects upon their outline and venation. 



VII. Where the size of the leaf and the complexity of its branching are consider- 

 able, as in most adult leaves of Leptosporangiate Ferns, the primordium shows 

 continued apical growth. The lower pinnae are then as a rule initiated monopodially, 

 budding out laterally below the apex. This adult condition is gradually led up to 

 in the successive leaves of the individual plant. But there is also as a rule a pro- 

 gression in the individual leaf from this adult mode of origin of the lower pinnae 

 to a sympodial origin, and finally to equal dichotomy.!" This is in fact a 

 reversion in the later stages of the ontogeny of the leaf to that branching which is 

 characteristic of the juvenile leaves. If the phyletic history be truly reflected in the 

 steps of increasing complexity of construction in the successive leaves from the 

 juvenile to the adult, then the steps in succession of origination would read thus: — 

 (i) Equal Dichotomy, modified to produce (ii) the Sympodial Dichotomy; and 



* Campbell, Eusp. Ferns, fig. 122. t See Land Flora, p. 627, and refs. 



