THE STIITLKS 17:i 



placed, tlu' cduvs vcrticjil. In ;i IVw leaves, tlie surfaces are normally 

 in the latter i)<)siti()n. Between the (h)rsal and ventral surfaces, there 

 are usuall\- (hll'erenees sufficient to necessitate their description sepa- 

 rately. In such descrii)ti()ns, it is better to speak of the dorsal surface 

 as being underneath rather than "helow," as the latter term may 

 confuse it with the basal region. 



Anatomical Elements of the Leaf. — It has been shown that the leaf 

 originates and develops as an extension of tlie peril)lem, covered by 

 that of the dermatogen, and that it develo])s a stele which becomes 

 continuous with that of the stem. In other words, its mode of develop- 

 ment is precisely like that of a stem-branch. We have in it, therefore, 

 all the elements which characterize primary stem-structure. The 

 connection of the leaf with the stem is usually by a s})ecially arranged 

 and constructed tissue, forming a distinct organ, the pulvinus, which 

 provides an articulation designed to afford a prompt and ready separa- 

 tion of the leaf at the conclusion of the performance of its function, 

 as well as for certain movements and changes of position during life. 



Just as branches of the cauline stele pass into leaf and branch, so 

 do those from the foliar stele pass laterally into its expansions, and 

 secondary and tertiary ones successively pass from them. These 

 branches are \ery frequently joined at their distal ends to others (Fig. 

 525, o), as well as at their proximal ends to the parent system. Whether 

 such is or is not the case, the result of the branching is the production 

 of a framework or skeleton which forms a support to the parenchymatous 

 tissue which fills its meshes and co^•ers its surface, the latter being in 

 turn covered by the epidermis. 



Except as to the general characters which follow under leaf-classi- 

 fication, it is im])ossible to ascertain the structure of the cortex and 

 epidermis of the leaf by ordinary methods, so that this subject is 

 relegated entirely to the (lei)artment of histology. 



The Stipules. — Before ])roceeding to the study of the leaf-blade, 

 which specially concerns us, certain peculiarities of the stipules, and 

 of the petiole, may be considered. The original function of the stipules 

 was probably to afford a protective covering to the bud. While this 

 function still persists, it is doubtful if that of increasing the foliaceous 

 surface has not come to be of greater importance. We should, therefore, 

 expect them to develop tissues and forms resembling those of the leaf- 

 blade, and such is the case, making them subject to the same classifica- 

 tion and terminology in those directions as will be ai)plied to the leaf- 

 blade. They ha\'e, however, certain peculiarities of their own which 



