68 PROCEEDINGS OF THE ACADEMY OF [1889. 



a part of the season, repeat this method of growth several times in 

 the course of six or eight feet. At each point where the root comes 

 to the surface begins later the development on its upper side of the 

 so-called " knees." 



A remarkably rapid increase of cells on one side takes place at 

 this point, which results in the constant elongation of the club 

 shaped body, the knee. This point of growth I shall discuss in a 

 following article. 



The second method of knee formation takes place on old roots 

 either horizontally or otherwise disposed to the surface. 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. 



In the first case the whole root becomes widened, ribbon like and 

 corrugated in general appearance. In the second, separate and 

 distinct knees are formed. All knees cannot, however, be explained 

 from these two simple methods. The cypress roots seem to have 

 a very great tendency toward natural grafting. Whenever two roots 

 cross each other and later through increase in diameter press upon 

 each other they develop a natural union. In many cases when the 

 ascending and descending part of the root which forms the basis of 

 a knee approach parallelism they become later, through increasing 

 diameter, wholly consolidated in the formation of the knee. It may 

 happen that several knees begin their development within a few 

 inches of each other. In such cases they may later become consol- 

 idated into one. The external parts of the knee above the soil in such 

 cases may give little or no evidence of such consolidation. In the 

 first mentioned method of knee formation the root passing from the 

 tree to the " knee " is always less in diameter than the one leaving 

 the knee. On the root descending from the forming knee there gen- 

 erally develops a cluster of roots, these often become consolidated 

 later with the " knee." Roots which branch from the ascending part 

 of the forming " knee " develop new " knees." 



I do not propose at this time to discuss 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. 

 Given conditions of sufficient dryness and plants of Taxodium 

 distichmn may be produced without a sign of these excrescences. 

 The same or other plants may be placed under conditions of extreme 

 moisture when after a time the " knees " will appear on both old and 

 newly formed roots. The Taxodium seems to prefer swamps and 

 inundated locations and in such places always produces the " knees." 

 When cultivated in gardens and parks in dryer soils, where it readily 

 grows, it never produces them. 



The possibility of causing the development of these aerating 

 organs in the cypress or, by changed conditions, of making them fail 



