158 



STRUCTURE OF COMPOUND LEAVES. 



parts appear on them, and these are of course much smaller and 

 at the same time more numerous. A leaf in which this is the 

 case is termed tri-pinnate. Such forms are generally peculiar to 

 different kinds of plants ; but there are some in which we find 

 them strangely intermixed, so as to display their real origin and 

 character. 



238. Such an example is afforded by the GleditsiaoT Honey- 

 locust tree of North America, known to English gardeners by 

 the name of the three-thorned Acacia. As in other Acacias, 

 the leaves are compound ; but the division often proceeds to 

 such different degrees, in different parts of the same leaf, that it 

 is difficult to say, whether it is to be considered pinnate, li-pin- 



nate, or tri-pinnate. Of 

 such a leaf, in the accom- 

 panying figure, the cen- 

 tral stem is the midrib ; 

 and from it proceed the 

 secondary veins on each 

 side. The highest pair of 

 these, bears on one side a 

 leaflet, which shows in- 

 dentations marking a ten- 

 dency to subdivision ; and, 

 on the other side, a series 

 of secondary leaflets, form- 

 ed by the complete sub- 

 division of the first. The 

 second pair of veins bears 

 on one side a series of secondary leaflets nearly similar ; but two 

 of these are seen to have again subdivided into more minute 

 leaflets : the distribution of the veins in which, however, pre- 

 cisely corresponds with that of the larger ones, so that a skeleton 

 of the whole would exhibit little difference in its several parts. 

 On the other side, a portion of another series of secondary leaflets 

 is seen ; but towards the extremity they merge again into a 

 larger leaflet. Below these again, we have a complete pair of 

 larger leaflets. If the whole of the leaf had been formed on this 



FIG. 67- LEAF OF GLKDITSIA, one of the Acacia 

 tribe, showing curious varieties in the subdivi- 

 sions of the leaflets. 



