64 



LEAVES. 



Sagittaria (fig. 51) 

 and Asarutn (fig. 

 52.) The former 

 an Endogens the 

 latter an Exogens. 

 From the above re- 

 marks the student 

 will readily deter- 

 mine the causes of 

 every variety of 

 form of simple leaves 

 which may fall under his observation ; arising, as the diversi- 

 ty does, either from the arrangement of the veins, or the de- 

 velopment of the parenchyma. The non-develpment of pa- 

 renchyma seems, by examination of some leaves, not to be 

 the cause in all cases of lobed leaves, but in some instances 

 the non development of the veins ; for the undulating margin 

 of such leaves shows an excess of parenchymous substance, 

 and yet the leaves are lobed. Example of this formation may 

 be found in different varieties of the oak, as the Quercus al- 

 ba, Lyrata, &c. The student will observe that the spaces 

 between the lobes, particularly in the Lyrata, are very spar- 

 ingly supplied with veins, showing that it was a want of de- 

 velopment of veins that caused the lobed form of the leaf. 



70. Compound leaves are those which have the lamina 

 articulated to a common petiole ; and this fact must be borne 

 in mind as the real distinction between simple and compound 

 leaves. No matter how much the lamina may be divided, if 

 the divisions are not articulated to a common petiole, the leaf 

 is simple ; and if the lamina is not divided at all, 

 but articulated to the petiole, the leaf is compound, 

 as in the Orange (fig. .53.) The principle of for- 

 mation of this class of leaves 

 will be readily understood by 

 the above explanation of sim- 

 ple leaves. 



All compound leaves may be 

 reduced to two varieties cor- 

 responding to the Feather vein- 

 ed and radiated forms of reticu- 

 lated leaves. If we recur to the 

 chestnut leaf given above, and 

 conceive each of the primary veins with 

 the secondary veins belonging to it, to 

 form a distinct lamina, we should iiave a true 



