TREE FERNS OP NORTH AMERICA MAXON. 473 



their attachment being nearly at right angles. The plants of which 

 figure B represents a trunk section were rather shorter and more 

 robust, with an immense crown of larger fronds rising from the 

 trunk like a huge funnel, their leaf stalks being suberect and in 

 attachment extending from the apex a distance of several inches 

 down the trunk. At the time these plants were collected they were 

 believed to represent two very different species. Nevertheless, 

 upon careful study subsequently it was found that they possessed no 

 technical differences whatever by which they might be recognized 

 as distinct, aside from the difference in shape and size of leaf scars, 

 even the scales of the trunk and leaf bases being exactly alike except 

 as to size ; and the conclusion was forced that tliese were forms assumed 

 by a single species, the differences noted in the trunks being dependent 

 upon the age and vegetative vigor of the plants. This conclusion was 

 substantiated later by an examination of several living West Indian 

 plants of this species in the conservatories of the New York Botanical 

 Garden, in which both kinds of scars were found to occur upon the 

 same individual, each in a separate zone. In one cultivated plant in 

 particular it was clear that a period of rapid vigorous growth, as 

 indicated by a sequence of long-elliptical distant scars, had been 

 followed by a period of diminished growth, during which the elonga- 

 tion of the stem had proceeded but slowly. The record of the latter 

 period was plainly written in the zone of closely set, rounded to 

 subhexagonal scars, the shape and position of these obviously having 

 resulted from the crowding of the leaves at the apex of the very 

 slowly lengthening stem. I have noticed a similar variation in the 

 rate of growth in other arborescent species of Cyatheacese, but none so 

 pronounced as in Cyathea arborea. 



The age of the larger tree ferns is difficult to determine, and I am 

 not aware that any authentic data are available. Certain specimens 

 in Jamaica have been reckoned 200 years old, which probably is 

 too high an estimate, although many individuals doubtless attain 

 an age of at least 100 years, if we may judge from the apparent rate 

 of elongation of the upper stem. Growth, however, is more or less 

 periodic at some stage in the life history of most large tree ferns, 

 depending upon favorable or unfavorable seasons and upon various 

 factors of environment; and the only reliable means of arriving at 

 the probable age of such individuals is from an extended series of 

 observations, over a long period, of plants growing in their native 

 environment. 



BRANCHING OF TRUNKS ADVENTITIOUS GROWTH. 



The upright stems of tree ferns are usually simple, but there have 

 been put on record several instances of occasional forking of the 

 trunk, and in a few species the tendency is rather pronounced. 



