r 58 Barber . — On the Nature and Development of the 
This layer of tissue forms the meristem of the thorn, and 
the further cell-formation becomes more and more restricted 
to this basal portion ; while the cells nearer the apex become 
rapidly elongated, pitted and thick- walled (Fig. 9). In a 
transverse section of a thorn at this stage it will be seen that 
the thickening of the cell-walls takes place to a greater extent 
towards the surface of the thorn than at its centre ; so that a 
mantle of hard mechanical tissue is formed around a softer 
core. This may take place to a much greater extent in 
other thorns than in those of Zanthoxylum alatam. Thus the 
hard persistent claw-like thorns of Caesalpinia Nuga are, in 
the specimens I have examined, perfectly hollow, — an ar- 
rangement quite in accordance with the laws of mechanical 
construction laid down by Schwendener. As illustrating the 
extent to which the thickening proceeds in thorn-cells, it is 
only necessary to macerate a soft thorn of Z anthoxylum. In 
this case it is easy to obtain cells in which thickening and 
pitting have rendered it impossible to determine the position 
of the original cell-cavity (Fig. 10). The further growth of 
the thorn until the autumn appears to be essentially the 
same in character as that already described. 
In sections of some of the thorns gathered at this later period, 
however, the merismatic regions appear to have become much 
more sharply defined (Fig. 11). It is seen upon examination 
that, in such cases, the merismatic cells are sharply marked 
off from the underlying cells of the cortex. They are much 
shorter and more closely packed than before. The meris- 
matic cells hitherto were isodiametrical, and the cells 
developing from them rapidly became irregular. The cells 
now being formed, on the contrary, assume and retain the 
brick-shaped character so frequently seen in those developed 
from secondary meristems. It is, furthermore, from the first, 
easy to distinguish the cells which owe their origin to these 
two kinds of merismatic tissue ; and the dividing line between 
them becomes more readily discernible as the thorn grows 
older. This is due to the fact that the cells on the two sides 
differ considerably in shape, and also in the character and 
