26 BULLETIN" 272, U. S. DEPARTMENT OF AGRICULTURE. 



the widely spreading and branching superficial root system that 

 has been more commonly recognized. The masses of roots at the 

 base of cypresses have proved such an expensive obstacle to the 

 digging of canals through swamps as to cause the financial failure of 

 numerous timber operations dependent upon this form of logging. 



CYPRESS KNEES. 



The peculiar cypress "knees," or upright conic portions of the 

 superficial root system, are of unusual interest. Knees occur mostly 

 where water covers the surface for long periods, and their height 

 corresponds rather closely to the average high-water level for the 

 locality. Occasionally they reach heights of from 8 to 10 feet above 

 low-water mark. Observations indicate further that in soft or 

 yielding situations the root system is much more extensive and the 

 knees correspondingly larger than in firmer soils. They appear in 

 general to be as common in alluvial as in nonalluvial swamps, and 

 in fresh as in stagnant or acid soils. The wood is light in weight, 

 but peculiarly gnarled and twisted in structure, and therefore of 

 extreme toughness. Except in their early stages, the knees are 

 usually hollow, and they commonly die at once when the tree is cut. 



In regard to the function performed by cypress knees very little is 

 known by actual experiment. Wilson, 1 in a series of carefully con- 

 ducted laboratory tests with cypress seedlings, found that when con- 

 tinuously submerged in water the superficial roots sent branches 

 upward to the surface. This suggested the function of knees as 

 breathing organs for the roots. In speaking of adaptations in marsh 

 plants, Warming 2 states that in some trees and shrubs erect roots are 

 developed which thrust their tips above water and convey air to the 

 root system beneath in the mud and water, and on the authority of 

 Kearney cites cypress as an example. At its lower end the knee has 

 a downward-reaching, pronged root system, smaller but very similar 

 in form and appearance to the main root system beneath the tree. 3 

 The knee thus might be termed a sort of secondary tree, with the 

 difference that it is composed of root wood. So far as is known there 

 is no record of a knee budding or sprouting. 4 It clearly is not devel- 

 oped for the purpose of reproduction. 5 



Although commonly credited in literature with serving the func- 

 tion of aerating organs, there is no definite proof from mature trees 

 of the part played by knees in the economy of the tree. An experiment 



1 Wilson, W. P., formerly of University of Pennsylvania. Proceedings Academy of Natural Science, 

 Philadelphia, April, 1889; see also Forest Leaves, Vol. II, p. 100-111. 



2 Warming, Oecology of Plants, p. 186. 



3 Fig. 5 shows clearly this structure in trees photographed by Robert H. Lamborn. See Garden and 

 Forest, Vol. in, p. 20. 



4 Seedlings are occasionally found growing from seed which lodged and germinated in the hollows of 

 decaying knees. 



s See "Effect of submergence," p. 38. 



