648 CONSERVATIVE ORGANS. [LECT. XI. 



ing to the number or division of their parts: thus 

 it is termed forked (bipartita) when it is divided 

 into two points, or appears like two simple spines, 

 united near the base; as in two-spined Arduina, 

 Arduina bispinosa: three-pronged (tripartita), 

 when there are three points, as in three-t horned 

 Gleditchia, Gleditchia triacanthes ; and branched 

 (ramosa), when it is divided into many lateral 

 points. 



In respect of situation, thorns are caulinar, 

 petiolar, or foliar. They are said to be terminal 

 (terminates) when they are situated at the termi- 

 nation of a branch or shoot, as in Buckthorn, 

 Rhamnus catharticus ; axillary (axillares), when 

 seated in the upper angle formed by the petiole 

 of the leaf and the branch, as in the Lemon tree, 

 Citrus medico; superaxillary ( super axillares), 

 when a little above that angle, as in Gleditchia 

 triacanthes; and subaxillary (infer axttUxrcs) , when 

 in the opposite situation. 



The anatomical structure of the thorn is, with 

 a few exceptions, the same as that of the branch 

 on which it remains as a part, after the bark is re- 

 moved; and in many instances it appears to be 

 merely an abortive shoot, arising, as Malpighi 

 suggested, from defective nutriment; but this 

 cause can be regarded as occasional only, for, 

 were it general, every description of thorn would 

 entirely disappear under culture, which is not the 



