tg11] CHAMBERLAIN—CYCAD TRUNK 89 
The number of rings, counted 30 cm. above the ground in the 
6-meter specimen, was about 150. The number of crowns, at a 
very low estimate, was about 100. Any estimate of the number 
of cones would necessarily be very uncertain, the only data being 
that the first cone was borne when the plant was 1 meter in height, 
and that a piece 25 cm. in length, taken near the top of the plant, 
had borne 6 cones. Estimated only upon this data, the number 
of cones would have been more than 100, but the estimate is doubt- 
less much too high, because ovulate cones are comparatively infre- 
quent on small plants. 
The number of rings, then, does not correspond exactly to either 
the number of crowns or the number of cones, or to the number of 
both combined. It is certain that in some seasons a plant produces 
a crown of leaves but no cone; and that it may produce a cone 
and no leaves; and, further, that it may produce both a new crown 
and a cone the same season, or it may fail to produce either a crown 
oracone. It is quite probable that when either a crown or a cone 
is produced, a ring is formed, and that when both a crown and a 
cone are formed the same season, only one ring is produced. 
We are inclined to believe that a period of vigorous growth, 
which would result in the formation of a new crown or cone, would 
produce also a growth ring, and that seasons which pass without 
the formation of a crown or a cone would not be marked by growth 
rings, the mere alternation of rainy and dry seasons not being 
sufficient for the formation of a ring in Dioon. If the number of 
rings should correspond somewhat approximately to the number of 
seasons, we should regard the correspondence as a coincidence, 
the crown and cone production being the determining factor. Of 
course, it is well known that dicotyls in such localities have seasonal 
growth rings. 
In Dioon edule the growth rings present a very different problem, 
for it is certain that they correspond to neither the number of 
crowns, number of cones, nor number of seasons. At Chavarrillo,. 
a plant 60 cm. in height and 20 cm. in diameter showed, in a trans- 
verse section near the base, a zone of wood 15 mm. in width. The 
number of rings was about 20, but the age of the plant, at a very 
conservative estimate, could not have been less than 100 years, 
