396 C. B. Warring—Growth-rings in Kaxogenous Plants. 
marks in a climate of absolutely no variation. I have before 
me a section of es ee also presented by Dr. Willis. This 
tree, as is well n, grows in the muddy margins of tropical 
rivers and all sin the shores, forming dense forests even at 
seen ~ any trees an ywhe 
To dispel any vestige of ‘belief that seasons and these mark- 
ings are connected as cause = effect, I add shah the Cycads 
require several years to form o 
he consideration of these fnots leads to the chacluwie that 
these circles have their origin in cycles of activity and repose, 
implanted in the constitution of the plant, which would con- 
tinue to manifest themselves although there were no climatic 
variations—a conclusion strengthened by the experience of all 
who have attempted, by artificially equalizing the temperature, 
to make their plants bloom all the year. It is true that where 
seasonal variations exist, the successive stages of activity and 
rest are for obvious reasons synchronous with them, but they 
are not absolutely dependent upon them 
We may conc se ‘too, that the pre-glacial flora sig ate 
similar cycles of growth ‘and rest, some of which may hav 
been of short Rnration measured perhaps by weeks, like diode 
of the Chenopodium, while others like ~~ Cycads may have re- 
quired several years for their completi 
The following propositions appear te ‘be established by the 
facts which have been presented. 
. Some exogens form rings at intervals much less than a 
ey "Others require intervals of several years. 
3. Some form no rings. 
- The presence or absence of rings in exogens occurs in all 
climates. 
5. Large and well defined rings are found under aces gaie 
in which there is absolutely no ~aesiienet variation of t 
aegis: or moisture throughout the 
seaso 
adopt any conclusion as to the inclination of the earth's axis 
which may appear to us most reasonable. 
