334 The American Naturalist. [April, 
ging up young trees, by cultivating the plant from the seed, and 
by observing exposed root systems. He finds that “if the tree 
_ requires, from inundation or other causes, more aerating surface 
than can be readily or rapidly produced by young and growing 
roots, then either the whole upper surface of the root in question 
may become more active and rapid in its growth, or the places of 
growth may be limited to certain definite points’—so knees are 
produced. He continues: “I do not propose at this time to dis- 
cuss the function of these knees, further than to say that their 
location and occurrence indicate beyond a doubt that they are for 
purposes of aerating the plant.” 
In the monthly publication of the Pennsylvania Forestry As- 
sociation (Forest Leaves, December, 1889), in a careful article on. 
the Taxodium, Prof. Wilson, referring to Prof. Shaler’s work, again 
adverts to the knees, and says: “ From recent experiments made 
by the writer at the department of Biology, it has been demon- 
strated that the knees are organs produced by the roots for the 
purpose of taking in a greater supply of oxygen than could 
otherwise be had from the surrounding water. 
This theory which finds in the knees and swollen boles of the 
cypress the sole function of conveying something advantageous 
from the atmosphere to the sap of thetree during periods of sub- 
mergence, seems to have been entertained as early as 1847, when 
Dr. Dickinson and Andrew Brown read before the Association of 
American Geologists, in Boston, a study of “ The Cypress timber of 
Mississippi and Louisiana.” In this interesting illustrated article 
of eight pages in Sz//iman’s Journal for January, 1848, they say, 
“ The cone-shaped, leafless protuberances, sometimes ten feet high, 
growing from the interlacing roots in a dense forest, resemble in 
all but their color the crowded stalagmites in some enormous cav- 
ern. By means of these protuberances the roots, though totally 
submerged, have a communication with the atmosphere. We 
suggest,” they say, “ that this function is fulfilled by the knees.” 
When this communication is cut off by the annual overflow ris- 
ing above the tops of the knees, the swollen base carries the simi- 
lar structure of the roots up the bole of the tree to an elevation 
sufficient to reach the atmospheric air. 



