VI] INITIAL CELLS OF LEAVES 109 



and this leaf-primordium. Thus in Polypodhmi vulgare each dorsal segment 

 is said to produce a leaf(75). But this is not of general application : more- 

 over even here leaves are not formed from the ventral segments. At first a 

 few rather irregular cleavages occur in the primordial leaf-cell. These lead 

 to the establishment of a two-sided initial, with one of its edges directed to 

 the centre of the apical cone, while the segments form two rows right and 

 left (Fig. 100, Bl). The principal walls will here lie as two zig-zag lines 

 approximately in the median plane of the organ. As the leaf thus produced 

 elongates, it takes a flattened form, and usually branches. The initial cell 

 maintains the same orientation, while the middle portion of each segment 

 forms part of a series of marginal cells. These undergoing repeated seg- 

 mentations by walls parallel to one another form a marked feature of the 

 developing leaves of Ferns (Fig. 107). In branched leaves the segmentation 

 at the leaf-apex does not determine the formation of the individual pinnae, 

 for the segments and pinnae do not correspond. There is reason to believe 

 that such degree of correspondence between segmentation and the formation 

 of parts as is seen in Ferns is not obligatory. The two phenomena may 

 coincide, but they appear to be causally independent of one another. 



The rare opportunity of observing apical segmentation in a fossil has been afforded 

 by Dr Kidston in a median longitudinal section 

 of a leaf-apex oi Zygopierts corrugata (Fig. 104). 

 The drawing shows that a regular segmentation 

 existed, possibly with a single initial cell. But 

 it is impossible to be sure whether or not a 

 plurality of initials may have existed in this 

 case. 



The leaf of most Ferns is a winged 

 structure, with a central rachis bearing 

 lateral flaps or wings. The development 

 of these is characteristically carried out 

 in the Leptosporangiate Ferns by the ^,. 



r\%. \o\. Zygopteris corrugata, \wA?,Voxizo\\tc- 



very regular and repeated segmentation tion (1985). Halifax Hard Coal. Collected 



of the marginal cells above noted. Trans- IjferSmTaTaf^^fx TssT''"" °^ '^'''''' 

 verse sections of the growing lamina of 



Scolopendrium give a striking demonstration how the whole lateral wing is 

 referable in origin to a series of such segments cut off alternately from the 

 sides of each of the row of wedge-shaped marginal cells (Fig. T05, D). 



The leading feature which characterises the Leptosporangiate as distinct 

 from the Eusporangiate Ferns is that in them each sporangium springs from 

 a single cell: this cell growing out from the surface-tissue undergoes regular 

 cleavages, which in themselves present a certain analogy with those of an 

 initial cell (Fig. 106). These will be described in detail later (Chapter XHl): 



