THE LEAVES OF VASCULAR PLANTS. 267 



certain number of plants which seem to have simple 

 leaves, but which may be considered as having com- 

 pound ones reduced to a single leaflet ; this evidently 

 happens when the common petiole is visible, as in the 

 Orange, but it does not the less take place when it is 

 absent or very short. This evidently appears to me to 

 be the case in the species of Genista and Cytisus, said 

 to have simple leaves. 



This shortness of the com-mon petiole is also remark- 

 able in another respect: when the leaflets to the number 

 of three, five, or seven, &c. arise from an extremely short 

 petiole, they then seem to do so in bundles ; we see this 

 in Aspalathus. In comparing together the species of 

 this genus, we find some which have the leaves unequally 

 pinnate, and a very distinct petiole ; in others it hardly 

 exists, the number of leaflets remaining the same. 



"When the leaflets are three in number, it is often 

 difiicult to decide whether the leaf forms part of the 

 system of pinnate or palmate leaves ; and most authors 

 .have left the question undecided, and have classed them 

 without examination among the palmate ones. The 

 only rule which I know to remove this doubt is this : — 

 When the three leaflets have their articulation situated 

 exactly at the top of the petiole, the leaf ought to be 

 considered palmate ; for example, Cytisus^ and most 

 species of Trifolium. When the common petiole is 

 prolonged beyond the two lateral leaflets, and the 

 articulation of the terminal one is more or less distant 

 from the other two, as in Medicago, Desmodium, See. 

 the leaf ought then to be considered as a pinnate one, 

 ^vith only two pair of lateral leaflets. 



There is a numerous class of pinnate leaves which 

 have an even number of leaflets — that is to say, in which 

 the terminal one is wanting ; they are said to be Ab- 

 ruptly Pinnate (pari sen abrupte pinnata); they 



