i3 2 
Notice of Book. 
bulged out by it. It is shown by the author that the curvature thus 
induced in the ray causes the cells of the ray to slide on those 
elements which adjoin it on its convex side. 
It is unnecessary to follow Dr. Krabbe in his consideration of the 
sliding growth of sieve-tubes ; the facts are here quite similar to those 
observed in the development of the xylem vessels. 
The cases next dealt with differ essentially from those just 
described, in so far as the sliding takes place in the longitudinal , 
instead of in the transverse direction. This occurs in the development 
of the tracheides, and of the bast and libriform fibres. It is here that 
the process is most obvious. Where, for example, a fibre of the 
secondary wood or bast grows to many times the length of the cambial 
cell from which it is derived, and that at a time when the elongation 
of the organ as a whole has long ceased, it is clear that mutual 
displacements must go on between the growing cells. This fact has 
been insisted on by many observers. The author shows in detail that 
these displacements necessitate sliding between the growing ends of 
the cells undergoing elongation. He further points out, that the 
process can only take place in a tissue which is still growing in the 
transverse direction. The tracheides or fibres insert their ends 
between cells, which are at first in contact with one another, and thus 
additional room is required, which can only be afforded by general 
transverse growth of the tissue. Dr. Krabbe shows that the small 
cell-lumina, which are found in transverse sections between the 
larger elements, represent the cut ends of intruding prosenchymatous 
cells. All elements, which thus force their way between those lying 
above and below them, must necessarily undergo changes of form in 
this process. 
It would have added to the interest of this part of the work if the 
author had been able to add figures of the fibrous elements at 
intermediate stages of their development from the comparatively short 
cells of the cambium. This would have given a much clearer idea of 
the phenomena actually involved than can be attained by the aid of 
reasoning alone. 
Although the occurrence of sliding growth is most evident in the 
case of the secondary tracheides and fibres, which are developed in 
regions no longer undergoing general elongation, there is no reason 
to doubt, that the same process goes on during the formation of the 
primary tissues. The author points out, that the disturbances actually 
