225 
Notice of Book . 
cylinder of the root as a single ‘ radial ' bundle. Secondly, it throws 
great light on the structure of the lower vascular plants, in which we 
may either have a single cylinder, not differentiated into individual 
vascu ar bundles at all, as in the simpler Ferns and in the fossil Lepi- 
dodendra, or a number of cylinders, each complete in itself. The 
confusion of these latter (under the name of concentric bundles) with 
the vascular bundles of the higher plants was another weak point in 
the anatomy of De Bary’s school. On these main questions it seems 
pretty certain that the anatomy of the future will follow the lines of 
Van Tieghem and Strasburger; the differences between them are com- 
paratively of trifling importance. 
A chapter on the connection of the vascular bundles as affected by 
the growth in length and thickness in the stem and root, does much, 
with the help of the diagrams, to clear up this rather difficult subject. 
It is pointed out that the protoxylem of each new shoot is continuous, 
not with the protoxylem of the next older shoot, but with its later- 
formed wood. In this way only can a continuous water-channel be 
maintained, for the protoxylem of the older parts will have already 
become disorganized and useless, at a time when a new shoot is 
formed. Similar considerations apply to the phloem. In the stem the 
new layer of thickening from the cambium, starts each year from the 
top, in immediate connection with the vascular bundles of the young 
shoots, and thence advances down the tree. The reason why the 
outer zones of secondary wood are always the most active in conduc- 
tion, is because these alone are in direct continuity with the youngest 
shoots and their leaves. So far as the wood is concerned this is 
modified by the presence of tangential pits, and similar contrivances ; 
in the phloem there is usually no such provision for communication 
between successive zones ; hence, as a rule, only the youngest layer 
of phloem, which is in direct connection with the leaves, is functional. 
This does not of course apply to Monocotyledons without secondary 
thickening, in which the same phloem may remain active for years. 
A section on the width and length of vessels introduces the strictly 
physiological portion of the book. On the former point there was 
not much to be done. The widest vessels, as De Bary had already 
shown, are always pitted vessels with short joints. The greatest di- 
mensions were found in a leguminous liane (Mucuna) where the 
vessels attain o-6 mm. in diameter. 
The determination of the length of the vessels was a more difficult 
