® 
1921] CHAMBERLAIN—GROWTH RINGS 301 
bundle is completely amphivasal, there is no cambium between the 
xylem and phloem, like that found in the primary bundles of many 
monocotyls (fig. 11). 
The xylem consists almost entirely of tracheids with bordered 
pits and with walls so thick and hard that sectioning is difficult. 
The cells cut off from the inner side of the cambium and not taking 
part in the formation of the bundles keep, more or less perfectly, 
their linear arrangement. They are short, somewhat rectangular 
in radial view, and are arranged in very definite rows (fig. 12). 
The tangential arrangement is not so regular (fig. 13). While 
they thicken only a little, they become thoroughly lignified and 
extremely hard, so that they 
add to the difficulty of cutting 
sections. They are marked 
by numerous small simple 
pits. 
The growth rings consti- 
tute the most striking feature 
of the stem. Der Bary, in his 
Comparative anatomy of vege- 
tative organs of the phanerogams and ferns, remarked that, while there 
seemed to be no reason why growth rings should not be formed in 
woody monocotyls, none had ever been observed. An examination 
of the literature of vascular anatomy failed to yield any account of 
such rings; but, to make certain that nothing had been overlooked, 
I wrote to Professor JEFFREY, and he not only informed me that 
such rings had never been reported, but also gave some suggestions 
which greatly facilitated the investigation. 
To the naked eye the growth rings are obvious, but under a 
16mm. objective no one would suspect their presence. In Dioon, 
where growth rings are obvious to the naked eye but not so 
conspicuous under the microscope, the rings are due to the fact 
that cells formed at the close of a growth period are somewhat 
smaller and have thicker walls than those formed when growth 
is resumed. In Aloe ferox the explanation is not so evident. An 
examination of fig. 14, showing three thick transverse sections, 
indicates that the rings can be seen, even in a half-tone reproduction. 
Fic. 14.—Aloe ferox: three thick trans- 
verse sections of stem; natural size 
