32 The Nature and Origin of Stipules. 



stipulate leaves which are developed by the elongation of the axial 

 portion alone. Sessile leaves also are of this type, hence the ab- 

 sence of stipules, the stipular tissue being incorporated into the 

 basal part of the blade. But even where stipules are present, the 

 lateral basal portions of the leaves are often in the closest 

 anatomical relation with the stipules. This may be seen in Viola 

 obliqua Hill (fig. 18) in which, near the bundle which passes into 

 the stipule, a similar one arises, takes its course up the petiole 

 supporting its narrow wing and is distributed to a small part of 

 the basal portion of the lamina. We shall find several cases 

 similar to this when we come to the consideration of the Rosacese. 

 There is in this a suggestion of the occasional separation of only 

 a part of the lateral portions to form the stipules and the incor- 

 poration of the remainder into the petiole and blade. 



The second case is that of the sheathing petiole as it occurs in 

 the Graminae, Aracese and Umbelliferse. In this case the central- 

 basal portion of the primitive leaf is very largely developed and 

 with it the lateral portions which form the margins of the sheath- 

 ing petiole. The lamina and true petiole are later developments 

 of the apical and axial tissues. We ai-e strongly supported in 

 this view by the fact that the sheathing petiole is interchangeable 

 with petioles of the ordinary type accompanied by stipules. This 

 occurs in the Umbelliferae. In Hydrocotyle and a few other 

 genera the sheathing petiole is wanting and stipules are present. 

 The closely related Aralia racemosa L. also has stipules. Still 

 more striking is the case of Comarum palustre L. in which the 

 basal leaves have the sheathing petiole remarkabty developed with 

 no indication of stipules (fig. 19), while the upper leaves possess 

 well developed stipules adnate for not more than half their length 

 (fig. 20). 



But the identity of the marginal tissue of sheathing petioles is 

 perhaps best shown in the Ranunculacese. In the upper basal 

 leaves of Ranunculus bulbosus L.,the separation of the lateral por- 

 tions is seen actually to have begun, presenting exactly the ap- 

 pearance of adnate stipules. The development can be clearly 

 traced from below upward. The first leaf has a short sheathing 

 petiole of the ordinary t}^pe (fig. 21). This is slowly modified 

 till in the fourteenth leaf (fig. 22) the vascular bundles have drawn 

 closer together, the sheath has grown shorter and the broad lateral 



