Corky Excrescences on Stems of Zantkoxylum. 1 5 7 
the bud, cut in paraffin with the microtome, exhibits the 
leaves in different stages of development (see Fig. 5 )- In 
such a series the successive leaves arise as protrusions on the 
stem following a well-marked spiral arrangement (phyllotaxis 
= f). The thorns are found on either side of the leaf where 
it unites with the stem, and the determination of the earliest 
stages of these outgrowths is greatly assisted by the 
constant presence of a lysigenous gland at the base of each. 
The first differentiation of the gland may be observed in 
about the third leaf from the apex. It consists of a small 
area of cells, with granular contents, staining deeply with 
haematoxylin, situated between the vascular ring and the 
epidermis (Fig. 6). Around this group the neighbouring cells 
are already assuming the concentric arrangement of a sheath. 
As, however, the gland becomes more distinct, there is an 
increase in the number of cells between it and the epidermis. 
And this increase of cells, accompanied, as it is, by the 
protrusion of the epidermis at this point, may be regarded as 
the first stage in the development of the thorn. 
In the thorn at the base of the next leaf but one, the gland 
is already fully formed, and the surrounding flattened cells 
form a many-layered sheath (Figs. 7, 7 a). A mass of tissue 
has been intercalated between the gland which is still close 
to the fibro-vascular bundle, and the epidermis. The latter 
may be seen to divide by anticlinal walls, and its cells, at 
this point, remain much smaller than the epidermal cells of 
adjacent parts. 
In a more advanced stage of development, the gland-cells 
have become disintegrated, and many of the sheathing-cells 
around have likewise disappeared (Fig. 8). The cells on 
each side of the gland have become collenchymatous, and are 
occasionally cut across by dividing walls. The thorn is 
prominent, and its cells have already begun to elongate in the 
direction of its axis. Cells in a state of division may be seen 
at various points, but these are now principally confined to a 
narrow band of tissue at the base of the thorn, just outside 
the collenchymatous cells surrounding the gland. 
M 2 
