1884.] Vascular Cryptogams and Gymnosperms. 63 



of the development of the leaf in a series of types of Vascular Crypto- 

 gams and Gymnosperms, beginning with those lowest in the scale ; 

 the third part points out the conclusions to be drawn from that com- 

 parative study. The chief facts and conclusions are as follows : 



In the simplest forms, the Hymenophyllacece, the apex of the young 

 leaf, is, according to Prantl, flattened, and has the two-sided apical 

 cell so characteristic of flattened organs ; it branches chiefly, if not 

 exclusively, by dichotomy. By the stronger development of one 

 limb of each dichotomy a sympodial arrangement is produced, 

 which appears in the mature leaf as an apparently well-defined 

 pseudo-axis or phyllopodium ; bat since the branching is dichotomous, 

 the phyllopodium is not clearly differentiated in the first instance 

 from the less strongly developed limbs. It is characteristically a 

 flattened structure, and is in most cases winged to its extreme base ; and 

 though the leaf is complicated, it has no peculiarity of conformation 

 at the base to which the term hypopodium could with reason be 

 applied. 



In the majority of Leptosporangiate Ferns, exclusive of the 

 Hymenophyllacece and Osmundacece, the apex of the phyllopodium still 

 retains the two-sided apical cell so characteristic of flattened organs, 

 but the branching of the leaf is at first monopodial, though there is 

 sometimes a return to the dichotomous branching in the higher 

 ramifications ; its apical growth is in some cases unlimited. Thus the 

 phyllopodium is here more clearly differentiated from the pinnae, the 

 structure of the apex is, however, still that characteristic of flattened 

 organs ; it is a winged structure to its extreme base, though the 

 wings are in many cases reduced to mere lateral ridges. 



In the Osmundacece the two-sided apical cell of the above-named 

 ferns is replaced, during the earlier stages of development, by a three- 

 sided, conical, apical, cell; and it is believed that in this respect they 

 are unique among vascular plants. The young phyllopodium is 

 thus typically a solid structure ; it is still winged, and its branching 

 is monopodial ; it shows peculiar modifications of form at the base, 

 but these are referable without difficulty to the fundamental winged 

 structure ; the apical growth is long continued, and in its upper parts 

 the phyllopodium becomes flattened as in other ferns. 



In Angiopteris, as an example of the Marattiacece, there is no single 

 apical cell occupying the bulky apex of the phyllopodium, but in its 

 place there is a group of four cells, and in this respect it approaches 

 the higher types of vascular plants. The phyllopodium is thus a 

 solid structure from the first, its apical growth is limited, and its 

 branching monopodial, though the order of succession of the pinnae is 

 here, as in other ferns, strictly acropetaL The peculiar stipular 

 structure at the base is referable in the light of similar structures in 

 Todea to a modification of the winged structure. 



