Anatomy and Physiology. 89 



A leaf is said to be sessile when it expands immediately 

 from the stem ; the portion next the stem is called its base, the 

 termination or farthest portion, the apex. When there is but one 

 piece, no matter of what shape, the leaf is simple ; when more, 

 it is compound ; a distinction, however, in some cases impossi- 

 ble to determine. In the latter case they are generally jointed, 

 and in the usual season fall off separately ; and this should be 

 remembered the more as the simple or compound leaf, which- 

 ever the case may be, runs through the genus, and in almost all 

 cases in the natural group. Goethe, the great German poet, 

 wrote a book, the Metamorphoses of Plants, to prove, among 

 other things, that the almost infinite forms of leaves with regard 

 to outline, &c, owe their origin to the various modes in which 

 the woody skeleton is ramified in the parenchyma. This distri- 

 bution is termed venation ; his theory is the one adopted by our 

 most learned botanists of the present day. The veins or bones 

 of the skeleton-frame either divide at the base, and run parallel 

 to each other their whole length, forming the parallel-veined 

 leaves, which are peculiar to endogenous plants ; or else there is 

 a back-bone which sends off others from it, which, in turn, meet 

 in all directions, called, from the resemblance to the meshes of a 

 net, netted-veined leaves, belonging almost entirely to the exo- 

 genous plants. All that now remains to a perfect understand- 

 ing of the varieties of these two grand divisions, will be disco« 

 vered at a glance by reference to the plate, where numerous spe- 

 cimens are presented for illustration. The leaf-stalk is called 

 the Petiole ; its expanded portion the Lamina or Blade ; the stalk 

 generally runs through to the end, forming the midrib. The 

 terms to express the names of the leaf are borrowed from the 

 names of different objects, which we will now proceed to ex- 

 plain. 



SIMPLE LEAVES. 



Fio. 1 . Orbiculate — Round, both diameters equal, circumference circular 



2. Reniform — Kidney-shaped, roundish and hollowed at the base, without 



angles. 



3. Cordate — Heart-shaped, ovate and hollowed at the base, hinder part no 



angles. 



4. Lunulate — Moon-shaped, round and hollowed at the base, hinder part 



no angles. 



5. Triangular — Three-cornered, disk surrounded by three prominent angles. 



