155 
know ledge of the Anatomy of Plants. 
correlated with a climbing habit. In some cases, as in the 
Bignoniaceae and Sapindaceae, this explanation certainly 
holds good. But there are many points in which it breaks 
down, as Herail has shown. Further investigation is urgently 
needed to enable us to understand fully the physiological 
significance of these highly modified structures. 
The recognition of the pericycle as a special zone of tissue 
immediately surrounding the bundle-system in both stem and 
root is due to the French botanist Morot. This layer, long 
known in roots as the ‘ pericambium,’ is of interest from the 
fact that it is the seat of new formation of tissue of all kinds, 
as of lateral and adventitious roots, internal periderm, and 
extrafascicular cambium. 
The time has, I think, come when we must return to the 
view that the vascular cylinder of the root should be regarded 
as a bundle-system (in some roots enclosing a pith), and not 
as a single bundle. The term ‘ radial bundle ’ may be con- 
venient, but it suggests a false comparison. The central 
cylinder of the main root is perfectly continuous with the 
bundle-system and pith of the stem, and in many cases the 
anatomical peculiarities of the one organ extend into the 
other. The same principle must obviously be applied to 
lateral and adventitious roots also. The change will involve 
some revision of our terminology, but it is necessary if the 
anatomy of the root is to be made clear. 
Before leaving the general structure of the vascular bundle, 
Haberlandt’s discovery that the finer bundles in the leaves of 
ferns are collateral, not concentric, may be noticed. This is 
interesting as helping to establish the correlation between 
collateral structure of the bundle and bifacial organization of 
the leaf. 
The question of the cause of annual rings in wood is too 
purely physiological to be dealt with here. We are still in 
want of a satisfactory explanation to replace that of de 
Vries, but we can hardly be wrong in correlating the forma- 
tion of new porous spring-wood in each year with the in- 
creased transpiring surface of the foliage. 
