268 VEGETABLE ORGANOGRAPHY. 



may have, like the preceding, the lateral leaflets either 

 opposite, as is most frequently the case, or alternate, 

 which has sometimes caused it to be falsely thought that 

 the last lateral one is an unequal one ; but it is always 

 distinguished from the true terminal one in its not being 

 at the top of the common petiole, which is prolonged a 

 little beyond it. 



The petiole may be prolonged either into a branching 

 tendril, as in Vicia, or into a simple process, as in Orohus, 

 or in spines, as in Astragalus, or (what is more singular, 

 and has not, I believe, been remarked,) in a true foli- 

 aceous limb ; this takes place in the Walnut, its leaf 

 is pinnate, with two or three pairs of lateral leaflets 

 articulated with the petiole, and that which has the 

 appearance of the terminal leaflet is a foliaceous ex- 

 pansion of the petiole into a true penninerved limb, 

 continuous with the petiole, and not articulated. This 

 phenomenon establishes a new connexion between 

 compound and simple leaves. 



A peculiar case of this class of leaves is where the 

 extremity of the petiole, which bears the lateral leaflets, 

 is prolonged into a foliaceous cup, hollow and funnel- 

 shaped ; I have accidentally observed this in the Pea 

 and Gleditsia. 



We have said that most abruptly pinnate leaves have 

 the leaflets opposite in pairs. The number of these 

 pairs is sometimes very great, sometimes ^very small ; 

 sometimes there is but one, as in Acacia diphylla. 

 When the petiole is prolonged in any form beyond the 

 origin of the leaflets, there is no doubt that the leaf 

 ought to be classed among pinnate ones ; but when it is 

 not prolonged it may be classed indifferently, either as 

 a pinnate leaf with one pair of leaflets, or as a palmate 

 one with two leaflets. The analogy of families where 

 this organization takes place makes me think that, 



