468 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
tant réle of conserving moisture by preventing radiation and the 
consequent drying out of the forest floor. 
Perhaps a majority of tree ferns, however, occur as an ieee 
part of the predominant forest ae their crowns often reaching 
nearly or quite to the level of the tree tops, or in not afewspecies 
even exceeding it; as, for instance, Cyathea pubescens, one of the tallest 
Jamaican species, which attains a height of 40 feet or more upon 
the heavily forested higher ridges of the Blue Mountains and easily 
thrusts its crown above the surrounding deciduous forest. There 
are also certain species, like Alsophila parvula, Cyathea furfuracea, and 
C. insignis, which in Jamaica grow indifferently in open and shaded 
situations, though their occurrence in the open may have followed 
naturally from the partial and piecemeal clearing of the land, the 
small cleared patches remaining under cultivation only a year or 
two before rapidly growing up to bush. It is noticeable that those 
individuals growing in the open often acquire a condensed or stunted 
form, as described later. 
At least one species, Cyathea arborea, flourishes in open situations, 
commonly in very large colonies. Jenman has described it in 
Jamaica as ‘gregarious, often covering acres on fully exposed slopes, 
everywhere shunning shade.” Perhaps on the latter account, and 
also because of its ubiquity, it is found more commonly than any 
other about dwellings and plantations, its huge, lacelike fronds 
lending an unusual decorative charm to scenes already novel and 
interesting to northern eyes. The formation of groves by this 
species in relatively dryish, open situations is almost unique for the 
family, although a few (e. g., Alsophila armata) are more or less gre- 
garious in partial shade, and many others of our American species 
are found in colonies in the deep, wet forest. In New Zealand the 
social tendency has even resulted in the formation of large groves 
under intensely humid conditions. One of these, which Colenso 
came upon in the forest called ‘‘Seventy-mile Bush,” in North 
Island, is described by him as follows: 
On a flat in the heart of the forest, in a deep hollow lying between steep hills, the 
bottom of which for want of drainage was very wet and uneven and contained much 
vegetable mud and water even in the dryest summer season, I found a large and con- 
tinuous grove or thicket of very tall tree ferns, chiefly Dicksonia squarrosa and D. 
fibrosa, with a few of Cyathea dealbata intermixed, with but few forest trees and shrubs 
growing scattered among them. I suppose they occupied about 3 roods of ground, 
and I estimated their number to be 800 to 1,000. They were all lofty, from 25 to 35 
feet high, and in many places growing so close together that it was impossible to force 
one’s way through them. 
Concerning tree-fern formations of this sort in both New Zealand 
and Australia, Christ has pointed out that they rarely consist of a 
single species, but are as a rule mixed associations of two or more 
