YampolsKY: Study of OU patm. IIQ 



Form of the dcvcloping leaflet, 



lu coiisidering the form of the leaflet we miist again return for a 

 moment to the origiii of the respective halves of the leaf blades. It miist 

 be remembered that they arise from that portion of the hook that grows 

 over and above the growing région in the form of a bonnet, the-edges of the 

 bonnet representing the respective halves of the leaf blade. Then the appear- 

 ance of the groove éliminâtes the leaf blade from the rest of the leaf. In cross 

 section thèse leaf blades (the respective halves) are triangular, the leaflets arise 

 from the base of the triangle which is the place of attachment of the leaf blade 

 to the rhachis of the leaf. Whereas in the initial stages of development, as 

 we hâve aiready seen, the leaflets are more or less sac-like in structure, 

 their outline very soon begins to conform to the cross-sectional outline 

 of the leaf blade — that is, they soon become triangular, flattened sacs 

 (Figures 17, 18, IQ, 20 and 21 Plate 111). 



The form of the developing leaflets cannot be explained without 

 taking into account the changes of position that thèse undergo as a resuit 

 of the development and growth of the other parts of the leaf and the 

 spécial relation that exists between parts of the leaf. 



The leaflets of what is to be the mature, fully divided leaf, arise as 

 parallel outgrowths, in a more or less horizontal plane. This is seen from 

 Figures 8, Q, 11, 12, 16, 17 and 18, Plate 11, as \nq.\\ as from those in 

 Plate 111. A cross section through a leaf blade in a very early stage of 

 development will show that to be true. Although the leaflets retain their 

 parallel relation to each other, the plane in which they lie begins gradually 

 to shift so that soon they are placed obliquely upwards. An examination 

 of the section of a séries of leaflets shown in Figure 24, Plate III brings 

 that oui. It also brings out the fact that the angles formed by the indi- 

 vidual leaflets with the vertical axis differ, that the upper leaflets become 

 more and more vertical and that between the lowest and the highest 

 leaflets différences in degree of angle occur. 



As a matter of interest it should be stated that shifting of the plane 

 of position of the leaflets happens very early so that a cross section of a 

 leaf in that stage of development will eut through two or more leaflets. 

 This is important in view of the erroneous interprétation given to such 

 sections by the older authors. 



The shift in the position of the leaflets can be ascribed to the 

 elongation of the rhachis, to the growth of the leaflets themselves, and to 

 the pressure of the newly formed leaflets at the base of the leaf blades. 

 The maintainance of the parallel arrangement of the leaflets is for the most 

 part due to the common insertion of their apices in the tissue of the 

 periphery of the original leaf blade, which structure still persists during 

 such stages as shown in Figure 18, Plate II where the long strip of tissue 

 to the right of the leaflets is the persisting strip of the young leaf blade. 



