1890.] The Knees of the Taxodium distichum. 339 
years, and who has just finished the largest drawbridge in America, at 
New London. Mr. Macdonald agreed with me that the root B, which 
is trussed with the knees C and C’, would very largely exceed in 
capacity for holding the tree firmly in yielding material the root A, 
which is similar but destitute of knees. This greatly increased security 
against destruction by storms is, I think, a sufficient advantage to ac- 
count for the existence and maintenance of an organ that draws so 
slightly upon the vitality of the plant. 
It is proper to record here another observation that may explain the 
existence of the elevated, narrow point which the knee sometimes 
develops, and which rises higher than the curved growth that would be 
necessary to secure the maximum resistance to compression and exten- 
sion. The home of the Cypress is in broad, level rivet-margins subject 
to periodic overflow, where hundreds of square miles become covered 
with a shallow bed of slowly moving water, or in basin-like depressions, 
sometimes of vast extent, where from time to time water rises above 
the level of the horizontal roots. Then these stake-like protuberances, 
rising into and through the current formed by the drainage or by the 
winds, catch and hold around the roots of the parent trees many 
thousand pounds of ‘‘ plant food’’ in the form of reeds and grass, or 
small twigs among which dead leaves become entangled. The tree 
that exclusively possesses this source of nutrition is at an advantage 
over all others in the neighborhood, and the higher these attenuated 
t drift-catchers ’’ rise in the stream, the more drift will they arrest, for 
the highest stratum of water is richest in float. The theory that some 
distinguished writers have suggested that the knee is a factor in the 
aération of the sap, and that the tree’s death is prevented by such 
aération taking place in the upper portion of the knee during periods 
of high water, would seem to need careful experimental confirmation. 
Where nature forms an organ whose purpose is to preserve the life of 
the individual, she takes special care to adapt such organ to the func- 
tion it is depended upon to perform. In this case the rough, dry bark 
of the knee offers a most imperfect means of access for the oxygen or 
other gases of the atmosphere to the interior vessels of the plant, and 
instead of presenting broad surfaces of permeable membrane, formed 
for transmitting elastic fluids, at its upper extremity the protuberance 
becomes more narrow and presents less surface as it rises, so that when 
during periods of high water the life of the tree is most jeopardized, 
the life-saving organ attains its minimum capacity. In the presence 
of this manifest want of adaptation it also seems important for the ac- 
ceptance of the aérating theory that some one should experimentally 
