GROWTH RINGS te A MONOCOTYL 
CONTRIBUTIONS FROM THE HULL BOTANICAL LABORATORY 285 
CHARLES J. CHAMBERLAIN 
(WITH SIXTEEN FIGURES) 
The principal object of this paper is to announce the discovery 
of growth rings in a monocotyl, but some observations upon growth 
rings in other plants may not be out of place. The most familiar 
example of periodic growth is seen in the annual rings of Gymno- 
sperms and dicotyls; but even when there is a strong tendency to 
form only one ring every year, there are numerous variations, 
especially when the rings are very wide. 
In Melia azedarach the annual rings are often more than a centi- 
meter in width, but it is common to find in each season’s growth a 
dozen or more secondary rings which are easily seen with the naked 
eye. The part of the ring formed in the spring and summer is 
quite sharply differentiated from that formed in the autumn, and it 
is in this autumn wood that the secondary rings are most con- 
spicuous. In the spring and summer wood the few rather indefinite 
secondary rings are due to varying proportions of tracheae and 
tracheids. The tracheae of summer wood are not very different 
from those of early spring; while in the autumn wood the larger 
cells merely start to develop into tracheae. They have transverse 
walls, which in some cases begin to break down, but here the 
development ends. The tracheids of the autumn wood are numer- 
ous and very thick-walled, so that this part of the ring is extremely 
hard. It is evident that the secondary rings are due to periodic 
acceleration and retardation of growth, which causes them to show 
some of the features characteristic of ordinary annual rings. 
Casuarina tenuissima affords another instance of numerous rings. 
A shoot about 3 mm. in diameter and only a few weeks old showed 
five or six well marked rings, due to an alternation of tracheae 
and tracheids. The plant from which the shoot was taken was 
293] [Botanical Gazette, vol. 72 
