1889.] NATURAL SCIENCES OF PHILADELPHIA. 69 



to appear at all, naturally leads oue to examine other plants which 

 from choice inhabit similar locations. There are numbers of trees 

 and smaller plants which when flooded part of the season or grown in 

 too wet soil will either form knees something like the cypress or send 

 their roots up into the air above the water. 



I have succeeded in causing common Indian corn to push up one 

 or more roots from each plant above the soil by keeping the same 

 saturated with moisture. Such roots grow up into the air and then 

 turn downward and enter the soil, forming perfect knees for aera- 

 ting the plant. 



In Georgia on slopes remaining inundated during the wet season 

 I have found the Pond pine, Pinus serotina Michx. making perfect 

 knees on the water side something like the cypress, while the roots 

 on the upper or dry side of the tree did not appear on the surface 

 at all. 



One of the most striking cases in which roots are sent up above 

 the surface of the soil and water may be found in one of the sour 

 gums the Water Tupelo, Ny.isa aquatica L. of the Southern states. 

 This tree sometimes grows in water holes associated with no other tree, 

 thus resembling a Cypress head. I found such Tupelo-heads fre- 

 quently in Georgia. In such cases the base of each tree was enlarged 

 to double the diameter five to eight feet from the ground. Around 

 the base of each tree extending six or eight inches above the high 

 water line was a compact mass of roots, each one growing vertically 

 up out of the water and after making a sharp bend growing down 

 parallel with the upright part into the water again. There were 

 sometimes dozens of these roots surrounding one tree closely appressed 

 to its base. These roots varied in size from that of the finger to 

 several inches in diameter. 



The genus Sonneratia, and also Avicennia L. both furnish interest- 

 ing trees which, growing in soils or ooze always saturated with water, 

 have contrived to send up vertical roots for purposes of aeration. 



Avicennia nitida grows in our own tropics and along the south- 

 ern shores of Florida. These vertical roots which extend up above 

 the soil from 6 to 10 inches are always in the air at low tide. They 

 are covered with numerous lenticells through which the air enters the 

 plant when they are not flooded. 



There is no doubt but that all swamp plants and others growing 

 between tide waters which are flooded during a part of the day 

 have provided themselves in one way or another with means for root 

 aeration. See interesting papers by K. Goebel 1 L. Jost, 2 and 

 Shaler. 3 



The following were ordered to be printed: — 



1 Berichle der rleutschen botanischen Gesellschaft, Jahrg. 1886, S. 249. 



2 Botanische Zeitung, Nr. 37, 38 u. 39— 1887— S. 601. 



3 Memoirs of the Museum of Comparative Zoology at Harvard College, 

 Vol. XVI. No. 1, June 1887; Science, Vol. XIII. No. 318 p. 176. 



