264 VEGETABLE ORGANOGRAPHY. 



prolonged into a spine, as in Coulteria; to the lobes of 

 leaves which have their nerves prolonged into spines, as 

 is seen in Thistles ; and to spiny teeth, which are only 

 lobes smaller than the preceding. The points of the 

 leaves of the Holly come under the class of foliary 

 spines ; but those of Aloe and Agave are analogous to 

 the lateral ones of petioles. 



Leaves, reduced to the state of scales, involucra, or 

 bracts, present similar phenomena, and approach, for 

 the most part, petioles devoid of limbs and prolonged 

 into spines ; this is readily seen on examining the 

 involucra of the Thistle and other spiny Compositae. 



Peduncles, like all the other organs, may harden at 

 the point so as to become spines. This takes place after 

 flowering, and is presented under two remarkable 

 forms : — sometimes the floral branches, more or less 

 ramified, remain after the fall of the flowers and fruits, 

 and form kinds of spines, usually branched, and ap- 

 parently terminal, as, for example, in Alyssum spinosum, 

 Mesembryanthemum spinosum, &c. ;— sometimes the 

 axis of the spike becomes indurated after flowering, 

 and is terminated, at maturity, by a hard point, which 

 in certain plants, as Trifolium subterraneum, serves, by 

 the curving of the peduncle, to penetrate the earth and 

 bury the seeds there. The pedicels, when they do not 

 bear flowers, are sometimes changed into spines ; this 

 appears to take place in Nauclea, Sec. 



The parts of the flower even, though more fugitive 

 than the others, and having consequently less time to 

 harden, occasionally present spiny degenerations. 



Thus, the sepals so partake of the nature of leaves 

 that they frequently terminate in spines, as in Stachys. 

 The spiny pappi of certain Composite come under this 



class. 



The petals themselves, notwithstanding their usually 



