THE LEAVES OP PLANTS. 



All these, and many other forms of simple leaves, depend upon the relative propor- 

 tions of development in the longitudinal and lateral directions ; for in every ease the 

 apex, or free end of the leaf, is first formed, and then the blade enlarges in both direc- 

 tions. As a rule, the growth proceeds more longitudinally than transversely ; and 

 thence, for the most part) leaves are longer than broad. But when it is equal in all 

 directions, the orbicular or rounded form of leaf results. Again, the lateral develop- 

 ment never proceeds equally from the base to the apex of the midrib, but is usually 

 greater at the former than the latter, thus constituting the ovate forms of leaves. In a few 



instances, however, the contrary is observed, as 

 in Figs. 160, 161 ; and it obtains the prefix 

 ob as obovate or obcordate. When develop- 

 ment proceeds regularly in these two directions 

 the surface of the leaf is flat, and may be farmi- 

 liarly represented by the palm, or aspect of the 

 hand with the fingers outstretched; but when 

 between any two veins or fingers the transverse 



development is uneven, a degree of puckerinsr 

 Fig. m-Obovate Fig. 161. -^cordate ^ ^ ag ^ ^ ^ Q holly fr 



a few instances of tolerably even growth, the 



resulting leaf is not flat, but somewhat tubular, as may be imperfectly shown by 

 contracting the hand so that the whole thumb and little finger shall approach eaoh 

 other. 



The most frequent variation is the arrest of development at the margin of the leaf. 

 If the lateral development were complete, it is clear that the edge of the leaf would be 

 even or entire ; but in many instances it is incomplete, and thence a deficiency ensues 



Fig. 162. 



Fig. 164. 



Fig. 165. 



Figs. 166, 167. 



Fig. 163. 



Fig. 162. Serrate leaf. 



Fig. 163. Doubly-serrate leaf. 



Fig. 164. A pinnatifid leaf. 



Fig. 165. A doubly-pinnatifid leaf. 



Fig. 166, 167. Hastate and lyrate-shaped leaves. 



which gires a tooth-like or crenate appearance to the edges (Fig. 162). Such a leaf is 

 termed serrated, toothed, or crenate. The extent of this deficiency varies much, and 



VOL. II. J*_ ^ 



