294 BOTANICAL GAZETTE [NOVEMBER 
growing in the greenhouse, and the number of rings corresponds, 
roughly, to the number of times the plant was watered thoroughly. 
Several years ago, in the neighborhood of Jalapa, Mexico, where it 
is rather rainy throughout the year, a species of Piper was noticed 
which showed no growth rings; while the same species, a few miles 
farther east, where there is a sharp alternation of wet and dry 
seasons, showed the anticipated rings. These are examples of 
immediate response to rather slight changes in conditions. At 
the other extreme are plants which show no response to seasonal 
conditions, but nevertheless are susceptible to stronger stimuli. 
Interesting growth rings which do not mark the number of 
years, but correspond to longer intervals, are found in the cycads. 
Dioon edule, after a period of coning or after damage by fire, loses 
all its leaves and goes into a prolonged resting stage which may last 
for several years. When it resumes activity and produces a new 
crown, a vigorous growth of wood takes place, with the formation 
of large tracheids, which, following the small tracheids of the 
nearly exhausted condition, produce a ring having the characters 
of an ordinary annual ring in Gymnosperms. These prolonged 
resting periods occur at long intervals, so that the number of rings 
would be of slight value in estimating the age of a plant. A 
section of the trunk of a specimen of D. edule-1.5 to 2 m. in height 
would enable one to estimate the interval between successive 
growth rings, since the approximate age of the plant could be 
determined; but at present it is not easy to secure such a section. 
In D. spinulosum the rings look like those of D. edule, but a ring is 
produced with the formation of every crown of leaves. Since 
crowns in this species are usually formed every other year, the 
number of rings indicates about half the age of the plant. In these 
cases the ring is a response to a change in conditions, but a very 
decided change is necessary to produce the result. 
It has long been known that some arborescent monocotyls, 
like Dracaena, Yucca, and Aloe, produce a distinct zone of secondary 
tissue surrounding the primary and derived from a meristematic 
region showing the characters of cambium. In 1912, while study- 
ing cycads in South Africa, I cut into a large plant of Aloe ferox 
