202 VEGETABLE ORGANOGRAPHY. 



Some examples will place these principles beyond 

 doubt, and will serve at the same time to make known 

 the varieties and origin of spines and prickles. 



The circumstance which most frequently produces 

 spines is the defective development of the branches of 

 certain trees which become indurated, and thus are 

 transformed into thorny points : thus, for example, 

 the spines of Prunus spinosa, the common Sloe, are 

 evidently only indurated branches ; they spring from 

 the axils of the leaves like branches, and frequently 

 bear leaves ; their structure is absolutely that of 

 branches; moreover, when one of these plants is found 

 in a very dry soil, there are more spines, or, in other 

 terms, many more abortive branches ; if, on the contrary, 

 it grow in fertile land, it loses its spines — that is to say, 

 all its shoots, instead of being abortive, are prolonged 

 into true branches. It is on account of this circumstance, 

 common to several trees and shrubs, and especially 

 those of the Rosacea?, Amygdalacese, &c, that we often 

 observe that spiny plants lose their thorns by culture, 

 as I have observed in the wild Medlar, in which all 

 the spines disappeared in two years. The spines of 

 Gleditsia, which are so enormous and branching, those 

 of Genista, Cytisus, and a multitude of others, are only 

 abortive and indurated branches. We might say that 

 these are ramal spines. 



The petioles of some species of Astragalus, of Hali- 

 modendron and Ammodendron, present an analogous 

 phenomenon ; they harden at the end of the life 

 of the leaflets, and when these fall off, or are ready 

 to do so, they change into very hard and sharp true 

 petiolary spines : from the nature of their origin, they 

 are always simple ; they become almost as hard as the 

 stem itself; for all petioles which have this spinescent 

 tendency are continuous, and not articulated at the base. 



