

62 



LEAVES. 



sily distinguish them by observing the net 

 work arrangement between the ribs. — 

 Falsely ribbed leaves, are those whose 

 primary veins unite near the margin and 

 form what appears to be a true vein, but in 

 reality consists only of an accumulation 

 of the extremities of the primary veins. 

 The other forms of simple reticulated 

 leaves, are those which have generally no 

 midrib distinguishable from the other 

 veins, but the veins as thev enter the la- 

 mina of the leaf radiate from their point 

 of entrance at the margin of the leaf in every direction, and 



thus constitute the radiated form of re. 

 ^^ *• ticulated leaves. The leaf of the Heu- 



chera Americana forms a good exam- 

 ple of this variety, (fig. 43.) It is 

 sometimes the case that the petiole sup- 

 ports the lamina by being attached to its 

 center, and the veins radiate from this 

 point in every direction, forming the 

 Peltate leaf, as exhibited in Hydrocotyle 

 and Nasturtion, (fig. 44.) Professor 

 Gray well remarks that the secondary veins of radiated leaves 



are always disposed according to the 

 ^^ / \ \ — feather- veined method ; so that we may 



assume the latter as the type of the 

 venation in Exogenous plants, and 

 conceive a radiated leaf to result from 

 the union of several feather-veined 

 ones. 



68. The above are the most common 

 forms assumed by leaves when the 

 spaces between the veins are perfectly filled by the develop- 

 ment of the Parenchyma ; but it often happens that the pa- 

 renchyma is not sufficient to occupy all the frame work of 



the leaf, and leaves assume a great 

 ^5 -^W. variety of forms from this cause. Let 



us suppose that in a leaf like the Heu- 

 chera, no more parenchymous substance 

 had been developed than was sufficient 

 to cover the veins themselves and we 

 should have the leaf of the Ranunculus 

 Panthorix, (fig. 45.) Should the leaf 

 have the same venation as in the pre- 



