58 



The Living Plant 



\iii. ^:j^ 



line with the petiole at one end and the veins branching off pin- 

 nately from an axial mid-rib. This shape and venation are 

 actually realized in the leaves of some trees, very typically in 

 Chestnut (figure 11, d), Elm, Rubber-plant, and Banana. Much 



oftener, however, this outline is 

 modified by a condensation of 

 the green tissue towards the 

 base of the leaf, which ensures 

 a shorter path of conduction 

 for water and the photosyn- 

 thate, while lessening simul- 

 taneously the weight and lev- 

 erage on the petiole. Such 

 leaves are necessarily of ovate 

 outline, and these ovate-pinnate 

 leaves are very common in na- 

 _,,.,, . ture. The shape is well typi- 



FiG. 14. — The theoretical form, seen in ver- 

 tical section, of an evergreen tree. Further fied in the Catalpa, for CX- 

 particulars in the text. i / r> -< i \ i • 



ample, (ngure 11, e), and is 

 represented in conventionalized form in our figure 13, c. In 

 some plants the condensation goes so far as to make the leaf al- 

 most round, as for example in the Red-bud (figure 11, i), when 

 the venation makes some approach to the palmate type and the 

 petiole is apt to be notably long. Such leaves often show a bulge 

 of the tissue downward each side of the petiole, thus displaying 

 a transition to the typical round shape with which we began. 



It is thus evident that three fundamentally-distinct condi- 

 tions of leaf exposure exist, with three corresponding types 

 of leaf shape, — the round-radiate, the linear-parallel, and the 

 ovate-pinnate. But innumerable intermediate conditions of leaf- 

 habit exist, and therefore innumerable intermediate leaf shapes 

 occur. These shapes have a large practical importance in the 

 classification and description of plants, and accordingly have been 

 named for this purpose with very great accuracy; and it is inter- 



