OF THORNS. 263 



The stipules of several plants become so indurated as 

 to present the appearance of true stipulary spines: such 

 are those of Pictetia. But it must be observed, that 

 the coussinets, or protuberances of the branch upon 

 which the petioles are placed, sometimes acquire so 

 large a lateral development as to form true spines, 

 which have often been taken for stipulary ones : they 

 are very well seen in certain species of Acacia, where 

 they coexist with the true stipules, in A. Hcematomma 

 for example ; but when only one of these two organs is 

 met with, it is almost impossible to affirm whether the 

 spines on both sides of the leaf are indurated stipules 

 or lateral expansions of the coussinet. Analogy with 

 neighbouring plants can alone remove the doubt. 



It happens in a small number of cases that the 

 leaflets are wholly or partially abortive, and the petiole 

 is changed into a spine, which is simple when all the 

 leaflets are abortive ; trifid, when the two stipules 

 adhering to the base of the petiole, or the two lower 

 leaflets reduced to the middle nerve, harden, forming the 

 two lateral branches of the spines ; quinquefid, when 

 the stipules and leaflets are present at the same time. 

 It is in this manner that the spines of different species 

 of Berberis appear to be formed, being evidently 

 nothing but leaves reduced to their middle nerve, the 

 bundles of axillary leaves replacing their physiological 

 functions. 



The leaf itself may be transformed into a spine in 

 two ways : sometimes it is found reduced to a foliaceous 

 petiole, more or less dilated, and terminating in a spiny 

 point, as appears to take place in Littcea, Yucca, &c. ; 

 sometimes the limb itself is prolonged into a spine 

 formed by the prolongation of the middle nerve, as in 

 Chuquiraga. What I have said of leaves is equally 

 applicable to leaflets, the middle nerve of which is 



