160 Barber . — On the Natitre and Development of the 
common base, while in other cases the irregularity becomes 
more pronounced. Such irregularities may be readily ex- 
plained on the assumption that the phellogen of adjoining 
cones has fused, so that the thorns, at first separate, have 
subsequently been raised upon a common base. The various 
degrees of irregularity observable in the bark of Zanthoxylum 
Clava-Herculis , for instance, are probably due to this cause. 
(See Figs, i and 2.) 
2. In other cases, such as Acacia pentaptera , the thorns are 
never raised upon isolated cones, but upon ridges extending 
the whole length of the plant. In this Acacia the stem has a 
star-shaped transverse section. The end of each ‘ ray ’ of the 
star is capped by a fibrous mass, and outside this, upon the 
edge of the stem, is a ridge of cork with thorns at intervals. 
A similar state of things is observable in Euphorbia lactea. 
The stipulary thorns appear upon corky ridges, three of which 
are met with in the transverse section. 
3. The sharp thorns in a specimen of Erythrina lithosper- 
ma , brought from Ceylon by Mr. M. C. Potter, have hard, 
rounded, stony bases, and are readily detached with their 
bases from the decaying bark. The tenacity of the latter 
may, no doubt, be very different in the living state. 
4. A number of thorns with corky bases have a further 
timbers in foreign countries. On the other hand, the anatomical portion of the 
work requires a careful study of the literature of thorns and of cork. 
Up to the year 1873, no work of great importance appeared upon the anatomy 
or morphology of thorns, although many writers had published short notice con- 
cerning them. In the following two years, however, a sudden interest was 
awakened in these structures, and half a dozen works of considerable merit and 
exhaustiveness appeared. Of these, Delbrouck’s paper in Hanstein’s Abhandlungen, 
ll, 1875, is the most important. In it the author carefully summarised the work of 
some fifty previous observers, and dealt with the whole subject, with copious 
illustrations of the anatomy and development of thorns. Since that time no work 
of any completeness has appeared. In none of these various papers can I find any 
reference to the cones in question, although several notices appear of the cork- 
formation in thorns. 
The literature of cork-formation is more extensive, but, with few exceptions, the 
papers deal with the ordinary cork of our dicotyledonous trees and shrubs. 
Perhaps, of recent papers, the nearest to the present subject is that of Miss Gregory 
on the development of corky wings in certain trees of North America, published in 
the Botanical Gazette, 1889-90. 
