RESPIRATION IN THE ROOTS OP SHORE-PLANTS. 331 



favourable localities it attains the height of one hundred, and one 

 hundred and twenty-tive feet ; its trunk is straight, clean, desti- 

 tute of branches for the greater part of its height, with a flat, 

 spreading top. The diameter varies from four to ten feet. There 

 was, a few years ago, in the Bartram garden, near Philadelphia, a 

 cypress tree one hundred and twenty-tive feet high, and twenty 

 feet in circumference. It is a peculiarity of this tree that when 

 it gets large its I'oots throw up conical protruberances, known in 

 the South as " Cypress knees." These are from one to two feet 

 high, and four or five feet across at the base, and always hollow. 

 They are covered, like the roots, with a small reddish bark. 

 Wliat use these are to the tree, or what causes them, is not 

 known. Michaux states that he never succeeded in causing them 

 to throw out shoots by wounding them. They appear in great 

 abundance on trees subject to annual inundation, and begin to 

 appear when the tree is about twenty-tive feet high. The "knees " 

 are sometimes produced on cultivated trees. A notable case has 

 occurred at Sion House, England. In this case the knees extend 

 sixty feet from the trunk. The base of this tree, and the 

 numex'ous " knees " which have sprung up about, are shown in 

 the engraving (PI. XXVII?) The "knees" are cut off and 

 used by the negroes at the South for bee-hives, buckets, and, 

 when provided with covers, they serve as boxes in which to 

 store various articles. The trunk in large trees is often hollow 

 for some distance above the ground, and in felling the trees it 

 is customary to build a scaffold, upon which the choppers stand 

 to cut the trunk above the hollow portion." 



It is ditiicult to say contidently that these " knees," one to two 

 feet high, have the same function as the breathers of our shore- 

 plants. 



On comparing the roots of Bruguiera Rheedii, a photograph of 

 which I show, we notice a great resemblance. This mangrove 

 throws up at a distance from the stem of the tree, " knees " which 

 are covered with lenticels, and no doubt serve the same function 

 as the breathers of Avicennia and Soymeratla. 



They are hard woody projections, seldom above a foot high on 

 the roots of trees growing at the mouth of the Brisbane River, 

 with the bark frequently abraded at the highest point of the 

 " knee " by drift-wood. 



Dead " knees " when the bark is decayed away, may be kicked 

 over and are much of the form of the head of the Cassowary. 



I cannot detain you with any further description of these 

 organs. The study of them is to be commended to botanists and 

 may establish the respiratory value of the lenticels, and the 

 necessity of keeping these organs open and free from lichenous 

 incrustrations in our peaches and apple-trees. 



