Corky Excrescences on Stems of Zanthoxylum. 161 
peculiarity in that a layer of corky cells of some thickness is 
continuous over the surface of the thorn. This continuation 
of cork-layers over the thorn is a phenomenon readily notice- 
able in many plants in botanical gardens, and it is by no 
means confined to thorns with corky bases. In specimens of 
Trevesia , if the thorn is pressed by the finger, a cap of whitish 
transparent tissue becomes detached, leaving behind a conical 
core of bright green colour. The cap is formed by sub- 
epidermal cork-layers ; and the twofold function of the thorn 
for purposes of assimilation and of protection is at once 
evident. Among the plants in which this peculiarity may be 
observed are Leea hori'ida (Ampelidae), Eriodendron anfrac- 
tuosum (Sterculiaceae), Erythrina,' insignis (Leguminosae), 
Aralia Maximowiczii (Araliaceae), — plants belonging to widely 
different orders. 
5. The development of cork in the stipulary thorns of Eu- 
phorbia splendens is, according to Mittmann, peculiar. He 
states that, at the base of the thorn, there are 4 to 8 
layers of cork-cells below the epidermis. At some distance 
from the base, a thick-walled lignified prosenchyma pushes 
between the epidermis and the cork-layers. As the base is 
left behind, these prosenchymatous cells increase in number, as 
also do the cork-layers. About the middle of the thorn, the 
6 to 8 cork- layers arch over and meet one another. Above this 
point the prosenchyma increases and the cork disappears. 
‘ The upper part of the thorn not only becomes dried very 
soon, but, when fully formed, becomes separated by several 
layers of cork-cells, from the lower part which still continues 
growing V 
6. Professor Balfour tells me that the thorns of Apocynaceae 
of the section Carisseae frequently form a corky separation layer 
at their base. The cases of Rosa , Robinia , and Cactaceae are 
referred to below. 
To explain the biological significance of cork-formation, 
1 Mittmann, Beitr. zur Kenntn. der Anatomie der Pflanzenstacheln, Inaug. Diss. 
Berlin, 1888. 
