34 DOUGLAS HOUGHTON CAMPBELL ON THE 



starch. Among the tracheicls near the ends of the bundle, there may somethnes be de- 

 tected a group of smaller and more rounded ones, — the jirimary tracheids. 



(PL 6, fig. 9. Inner end of a transverse section of the Imndle of the petiole: xy, 

 tracheary tissue; ^jA, phloem; sli, bundle-sheath. The band of trachcary tissue is not 

 as strongly curved as usual.) 



The first leaf developed from the end of a stolon shows striking embryonic characters, 

 the whole leaf resembling in many respects the early ones of the germ-plant. The groove 

 in the front of the petiole is not so strongly mai-ked as in the later leaves, the petiole 

 being nearly cylindrical ; the hypoderm is reduced to form two to three rows of cells, 

 and these are not so well differentiated as in the later leaves. The fibro-vascular bundles 

 differ much, resembling almost exactly those of the third and fourth leaves of the germ- 

 plant, being oval in section instead of elongated, and the elements of the bundle are less 

 perfect than in the later leaves. 



As is the case in the eai-ly leaves, the two bundles of the petiole in the mature leaves 

 of the plant unite in the rachis into a single bundle, by a straight band of tissue forming 

 so as to connect their inner edges. The bundle shows in its earlier stages four points 

 at which the foi'mation of tracheary tissue begins, corresponding to the same points in the 

 separate bundles lower down. A cross-section of such a bundle, especially when treated 

 with potash, shows clearly the position of the different elements of the bundle while 

 still incomplete. The central procambium cells, from which the tracheary tissue is to 

 form, become much clearer than the outer ones, and are seen to be, for the most part, 

 larger. The pi-imary tracheids appear almost simultaneously at the four points men- 

 tioned, and are easily recognized by their thickened Avails. 



Following these a number of small tracheids are formed around the border of the cen- 

 tral mass of procambium, the development proceeding thence toward the center. As in 

 the bundles of the petiole, the bulk of the tissue lying between the tracheids and the bundle 

 sheath undei-goes little further change, the sieve tissue being even less perfectly devel- 

 oped than in those parts. The development of the pinnae begins with the formation of 

 segments by the apical cell of the leaf, after the blade becomes differentiated. Each seg- 

 ment gives rise to a pinna, which in turn becomes deeply pinnatifid. The formation of 

 the pinna begins by the marginal cells of the segment growing more rapidly than those 

 lying- within them, thus causing the outer parts of the segment to project. These mar- 

 ginal cells multiply rapidly by longitudinal walls which repeatedly bisect them. Frequentl}', 

 by one of the marginal cells in the middle of the young pinna becoming divided by an 

 oblique instead of vertical wall, a triangular marginal cell is foi-med which becomes the 

 apical cell of the pinna and divides in the same way as the apical cell of the leaf. On 

 account of the difference in age of the segments from which they arise the pinnae are 

 not opposite, but alteimate. Whether or not an apical cell is always formed in each pinna 

 is questionable; at any rate it cannot always be detected in the pinna when first formed. 

 Sooner or later it is obliterated, in the same way as that of the prothallinm, by a trans- 

 verse wall, which is followed b}' a second wall at right angles to the first, the two mar- 

 ginal cells thus formed acting from this time on in all respects like the others. 



. In cases where an apical cell is formed in the pinna, the segment divides, as in the 

 apex of the frond, into two cells, an outer and an inner, the latter forming the midrib or 



