133 
Notice of Book. 
observed in the arrangement of the cells afford evidence for this. 
Careful comparative measurements, both of the growth of the whole 
organ and of the individual fibrous elements, are clearly necessary in 
all such cases. 
Dr. Krabbe then proceeds to discuss the development of the xylem 
in the secondary bundles of those Monocotyledons which are capable 
of indefinite growth in thickness. These bundles are formed from 
the cambium in parts of the stem in which longitudinal growth has 
ceased. Dracaena Draco is the first example considered. In most 
cases all the elements of a bundle seen in any transverse section arise 
from a single cell of the cambium. The bundles are here concentric, 
the small phloem being surrounded on all sides by the xylem. The 
latter contains some woody parenchyma, but is chiefly composed 
of very long tracheides. The formation of a new bundle begins with 
the appearance of longitudinal divisions in a cell of the cambial zone. 
The cells thus formed only differ from the cambial cells in their smaller 
diameter and in their transverse walls becoming slightly inclined 
instead of horizontal. These young elements of the vascular bundle 
may be termed, for the sake of clearness, the sub-cambial cells 1 . 
Now the length of these cells, which is very constant, is found by 
Dr. Krabbe to average o-i mm. The average length of the mature 
tracheide is 3-8 mm. Thus, as each sub-cambial cell, which becomes 
a tracheide, grows to thirty-eight times its original length, while 
there is no elongation of the organ as a whole, it follows that in any 
given transverse section the mature tracheides cut through will appear 
on the average thirty-eight times as numerous as they would have 
appeared before their elongation. Or, in other words, each mature 
tracheide will make its appearance in successive transverse sections 
thirty-eight times as often as the sub-cambial cell from which it is 
derived. The author has repeatedly counted the tracheides seen in a 
transverse section of a fully formed bundle. He finds that their 
number varies from thirty-two to forty-four, the mean thus being 
thirty-eight. On the author’s assumptions it is possible to calculate 
from these data the number of sub-cambial cells at any one level, 
which give rise to the tracheides. It may be convenient to give a general 
expression for his calculation, as it is applicable to all cases of longitu- 
1 This is not the terminology used by the author, but the terms adopted here 
will probably be more intelligible to English readers. 
