12 FAMILIES OF FLOWERING PLANTS. 
trees growing in such situations, and illustrates an adaptation to envi- 
ronment of which plant life furnished so many interesting examples. 
The purpose of the knees is not only to furnish the tree with props, 
4 {m 
Fi. 8. —Tamarack swamp with border of sedges. (From MacMillan’s ‘Minnesota Plant Life” 
by courtesy of the author). 
but to admit air to the roots, as is proven fy the fact that the trees 
are frequently killed when flooded above the tops of the knees. 
Tribe Cupressineae.—Nine genera, the species of which are very 
widely distributed. The most important are Callitris, with 15 Afri- 
can and Australian species; Zibocedrus, with 8 species in America and 
New Caledonia; Cupressus, with 12 widely scattered species; Chani- 
wcypauris, with 4in North America and Japan; and Juniperus, contain- 
ing 40 species, distributed throughout the whole temperate zone. Sev- 
eral species of Chamacwcyparis, particularly the Lawson’s cypress 
(C. Lawsoniana) and the yellow cypress (C. Wutkaensis) both occur- 
ring on the Pacific coast, are valued both as timber trees and as orna- 
mental shrubs in cultivation. Cupressus and Libocedrus also furnish 
valuable timber. An African species of Callitris, the sandarac tree, 
yields sandarac, which is a white resin, used both as an incense and in 
