The Nature and Origin of Stymies. 37 



those of the central basal portion. The lateral portions remain 

 of considerable breadth to the top where they are distinctly 

 rounded off, and their bundles, with the exception of two or three 

 of the exterior ones, curve around and unite with those entering 

 the petiole. This free condition of the exterior lateral bundles 

 with the anastomosing network between them shows a consider- 

 able degree of approach to the true stipular condition. 



In the third case true stipules are developed. They are formed 

 by a very early separation of the lateral portions from the main 

 body of the primitive leaf, a separation which can be very clearly 

 traced progressively in the embryological history of leaf develop- 

 ment. The function of the lateral portions in their primitive 

 connection with the main body of the foliar organ is, in common 

 with the other portions, protective, and while the apical portion, 

 having had placed upon it the special function of assimilation, 

 goes on in its development together with the accessory axial por- 

 tion in adaptation to this purpose, the lateral portions usually 

 serve their ancient function only, sharing it with the central-basal 

 portion when this has not disappeared by degeneration. The 

 central-basal portion also supports the main body of the leaf, a 

 function from which the lateral portions have been freed by sepa- 

 ration. 



It is in consequence of this separation that all the main vascu- 

 lar bundles of the leaf-trace in the third type of leaf-development 

 are deflected toward the central one that they may pass up through 

 the petiole into the lamina and give the required support to these 

 important parts. The support of the lateral portions is left to 

 comparatively small lateral branches from the two exterior bun- 

 dles of the trace, evidently developed expressly for the purpose. 

 This we may conclude, since vascular tissue is the most modern 

 of plant tissues and introduced because of the necessity of sup- 

 port in the evolutionary development of the primitive ground 

 tissues. It would, therefore, follow and not precede the evolution 

 of leaf-forms, being introduced where needed and disappearing 

 again when degeneration or other support of particular parts ren- 

 ders its presence unnecessary. This will appear in some of our 

 examples. In the first and second types of leaf-development the 

 lateral portions may retain in greater or less degree their inde- 

 pendent venation. 



Annals N. Y. Acad. Sci., X, May, 1897.— 4. 



