244 VEGETABLE ORGANOGRAPHY. 



4tli. It happens more seldom in some plants tliat the 

 leaflets are abortive, and that the petioles remain naked 

 and cylindrical, without elongating into tendrils, or 

 changing into a spine. This takes place, for example, 

 in Leheckia nuda, (PI. 10, fig. 5,) and Indigofera juncea, 

 which some, for this reason, have said to be Aphyl- 

 lous. 



5th. When in compound leaves the extremity of the 

 petiole does not bear a leaflet, it then frequently hap- 

 pens that, remaining soft, it is prolonged either into 

 a little process, or a true and branching tendril, as is 

 seen in Orohus, Vicia, Lathyrus, &c. ; sometimes even 

 all the lateral leaflets are abortive, and the leaf is not 

 composed of more than a petiole transformed into a 

 tendril, as in Lathyrus aphaca ; but in this example, as 

 well as in the Phyllodium, the leaves of the young plant 

 frequently present leaflets which are afterwards wanting. 



6th. Finally, in the same classes of compound leaves 

 where the terminal leaflet is absent, it often happens 

 that the petiole hardens into a spine at its extremity, as 

 is seen in Astragalus. I do not insist upon these two 

 last degenerations, as I shall revert to them in a more 

 general point of view in Book IV. Chap. I. and II. 



If we now consider in the same manner leaves with 

 simple nerves, or those of Phanerogamous Monocotyle- 

 dons, we shall find analogous facts. The structure of 

 their petiole, when it exists, is modified by the dis- 

 position of the fibres : they are always placed side 

 by side, so that the base of the petiole is more or 

 less sheathing ; above the base these fibres are some- 

 times united into a triangular or semi-cylindrical petiole, 

 as, for example, in several species of Hemerocallis, 

 Alisma, &c. In almost all the Palms we find also a 

 petiole nearly triangular, expanded at its base into 

 a kind of dry sheath, the fibres of which are very 



