STRUCTURE OF THE LEAF. 



79 



THE LEAF. 



It is possible to determine but very 

 little of the leaf structure without the 

 aid of a microscope. The gross parts of 

 the leaf have already been defined (see 

 Fig. 3) as the base, consisting chiefly of 

 a pulvinus, the stipules, petiole and 

 lamina. In addition to these we have to 

 note that many leaves possess a basal 

 portion which sheaths the stem, but which 

 is not exactly in the nature of a petiole 

 or of stipules, and is denominated the 

 Sheath (Figs. 385 and 38Ga). Leaves 



f«^,38S, 



possessing a sheath frequently bear at the 

 point of its junction with the lamina a 

 blade-like appendage, usually very small, 

 and analogous, if not homologous, with 

 the crown of a petal, denominated the 

 Ligule (Fig. 385b), It has also been 

 shown that the leaf originates and de- 

 velops as an extension of the periblem, 

 covered by that of the dermatogen, and 

 that it develops a stele (or more than one) 

 which becomes continuous with that of 



pass laterally into its expansions, and 

 secondary and tertiary ones successively 

 pass from the latter. These branches are 

 very frequently joined at their distal ends 

 to others (Fig. 388a), as well as at their 

 proximal ends to the parent system. 

 Whether such Is or is not the case, the re- 

 sult of the branching is the production of 

 a framework or skeleton which forms a 

 support to the parenchymatous tissue 

 which fills its meshes and covers its sur- 

 face, being in turn covered by the epi- 

 dermis. Bundles which obviously sepa- 

 rate from one another at or near or below 

 the base of the blade, and maintain their 

 course well toward the apex or margin, 

 are called Costae or Ribs if equally prom- 

 inent (Fig. 474). Nerves if lateral and 

 markedly less prominent than one or more 

 of the central ones (Fig. 412). 



The central one, whether there be others 

 or not. Is the Midrib (Fig. 387a). 

 Branches of ribs or nerves are called 

 Veins, and they are distinguished as Pri- 

 maries (b) when departing from the mid- 

 rib, Secondaries (c) when departing from 

 primaries, and so on. Primaries of lat- 

 eral ribs or nerves must be specially so 

 designated in description. When there 

 is connection at the distal ends (Anas- 

 tomosis or Reticulation Proper, Fig. 388a) 

 it is essential that the order (Primary, 

 Secondary, and so on) of the anasto- 

 mosing veins be stated, also the region of 

 the leaf where the anastomosis takes 



place, 

 lets. 



Very small veins are called vein- 



Except as to the general terms which 

 follow under leaf classification, it is im- 

 possible to ascertain the structure of the 

 cortex and epidermis of the leaf by or- 

 dinary methods, so that this subject is 

 relegated entirely to the department of 

 histology. 



Very rarely has the leaf a terete form 

 and a radial structure as seen in trans- 



the stem. In other words, its mode of verse section. Typically it is a flattened 



development is precisely like that of a 

 stem-branch. We have in itj therefore, 

 all the elements which characterize pri- 

 mary stem structure. Just as branches of 

 the cauline stele pass into leaf and 



body. One flattened surface, the Ven- 

 tral, faces upward or toward the stem 

 which bears it and is ordinarily spoken 

 of as the upper leaf surface. The under 

 or outer surface is technically known as 



branch, so do those from the foliar stele the Dorsal. By a partial twist in the 



