Harris — Thorns of Gleditsch>a Iriacanthos. 221 



thorn is sometimes placed immediately above the other, and 

 sometimes removed a considerable distance. Compare figs. 

 14, 17, 18, 19 and 22. This may be accounted for by the 

 variation in the distance above the axis of the leaf to which 

 the first formed bud is carried before growth in length ceases. 

 Even where a second branch is not produced considerable 

 variation in this distance is noticeable, the leaves being some- 

 times immediately, and sometimes a considerable distance, 

 below the branch. Compare the figures on Plate XXI. 



While I am not prepared to say whether the production of 

 leaves on thorns is more common one year than another, I 

 have been able to note, in a general wav, no differences in the 

 number of cases of superposed thorns produced in different 

 years, the occurrence of such being seemingly equally numer- 

 ous among the old, weathered thorns, which are falling off 

 the trees, and those more recently formed. 



It is certainly not without interest or significance that, on 

 the thorn produced from an adventitious bud, the branch 

 developing from the bud which probably corresponds to the 

 one producing the thorns on the normal twig is almost invari- 

 ably a perfectly formed thorn showing little variation in form, 

 while the one developing from the bud which probably cor- 

 responds to the one producing the foliage branch in the nor- 

 mal twig shows a considerable range of variation in form. 



It seems not at all improbable that it might be possible to 

 find a complete series of gradations between the most special- 

 ized type of thorn and the foliage branch produced from 

 adventitious buds. It is certainly difficult to determine to 

 what class some of the material examined belongs. Figs. 28 

 and 29 show a stem, bearing some resemblance to the usual 

 type of twig, developed from one of the supernumerary buds 

 on an adventitious twig, which otherwise would have passed 

 as a perfectly formed thorn. In Fig 30 is seen a well-formed 

 and branched thorn developed from one of the supernumerary 

 buds on a twig produced from an adventitious bud on the 

 trunk of a large tree. This occurrence was very common in 

 the twig from which this was taken, in one case three well- 

 formed thorns being produced. 



The production of the anomalies above described seems to 

 be confined, for the most part, to certain individuals, or at 



