Corky Excrescences on Stems of Zanthoxylum. 159 
thickness of their walls, and depending thereupon, in their 
hardness and general macroscopic characters. 
The cells now formed rapidly assume the appearance of 
corky tissue : they are found in sheets, and exhibit rings of 
growth exactly similar in appearance and nature to the rings 
of growth in an ordinary Finns- stem. The thorn is thus 
gradually elevated, and the £ corky cushion ’ first makes its 
appearance. There is a rupture of tissues around the base of 
the thorn. This appears, however, before the formation of 
cork : and is probably due to the great tension to which the 
epidermal cells are subjected by the rapid increase in size of 
the lower parts of the thorn, after the capacity for growth in 
the epidermal cells has become diminished. 
In more advanced thorns, selected at haphazard, from the 
bark of older parts of the tree, the cone has already reached a 
considerable size (Fig. 3). In rough sections the dividing 
line between the thorn proper and the corky base is now 
readily visible with the naked eye. This line of separation, 
observable in the first instance because of the difference of 
the form and the manner of thickening of the cells on either 
side, is now emphasized by the appearance of a split across 
the base of the thorn in this region (Fig. 12). 
The cells of these older thorns are softer and much more 
easily cut than in younger ones, and this decay, together 
with the split already noticed, probably leads to the later 
separation of the thorn from its more durable corky base. 
Such a separation accompanied by long continued cork- 
formation in these localised areas, undoubtedly leads to the 
formation of the accurately chiselled pyramidal excrescences 
of such frequent occurrence in older Zanthoxylum- stems. 
General Notes on Cork-Formation in Thorns 1 . 
1. While the excrescences upon a large number of stems 
are regular and conical, this is not always the case. Not in- 
frequently two or three cones may be seen to spring from a 
1 The literature of the subject is very extensive, including, as it does, in the 
first place, numerous works on descriptive Botany, besides special treatises on 
