1911] CHAMBERLAIN—CYCAD TRUNK 83 
indebtedness to these gentlemen, for without their cooperation 
the investigation of such inaccessible material would have been 
impossible. . 
MACROSCOPIC STRUCTURE 
The conventional account of the trunk is doubtless true for all 
young cycads and for most old ones, but it is not correct for large 
plants of Dioon spinulosum, and probably not for others which 
have attained any considerable height. In D. spinulosum the 
large amount of wood, the zone sometimes reaching a thickness of 
10 cm., first attracted my attention, but since material was avail- 
able, it seemed desirable to examine the whole trunk. Plants were 
studied both at Tuxtepec and at the Hacienda de Joliet, but the 
following account, whenever it relates to D. spinulosum, is based 
upon material from the latter locality. 
Acr.—As mentioned in a previous paper (7), the trunk some- 
times reaches a height of more than 16 meters. From the crown 
to the base the trunk is marked with a series of ribs due to the 
alternation of foliage and scale leaves, the constrictions between 
ribs corresponding to the scale leaves, and the ribs themselves 
being the larger leaf bases of the crowns of foliage leaves. Ob- 
viously, the number of crowns which a plant has borne can be 
determined by counting the ribs, and, assuming that a new crown 
is produced each year, the age of the plant would then be known. 
But it is not certain that new crowns are formed every year, and 
whether the interval is regularly two years remains to be deter- 
mined. At any rate, an estimate making the number of years 
_ correspond to the number of crowns would be extremely conserva- 
tive. In our previous account (7) the age of the tallest specimens 
was estimated at about 400 years, the estimate assuming a crown 
- to be produced every other year. It was also stated that the scars 
are so obscure on the lower portions of the trunk that accurate 
counting is difficult. While this is true, we now find that the count- 
ing can be carried much farther than we had supposed, and also that 
the obscure ribs formed by successive crowns are very much closer 
together in the lower than in the upper portion of the trunk. The 
difference between the upper and lower portions of a trunk 6 meters 
in height is shown in figs. 1 and 2, the lower portion being taken 
