EXTERNAL CONDITIONS ON PLANT LIFE. 167 



courses and in the swamps of the south-eastern part of the 

 United States. It seeds itself naturally nowhere outside of 

 areas which are for several months during the year under water. 

 This tree belongs to the Pine family, but has some very marked 

 peculiarities which make it differ strongly from any of its 

 relatives. 



The tree shown in Plate No. I stands normally surrounded 

 by water. Many root-like projections are seen underneath and 

 around the tree, which are popularly known as cypress knees. 

 These are all connected with the root system below the water. 

 They have extended their growth upwards until they are sure 

 to remain in the air at the ordinary level of the water during 

 most of the year. They are in such numbers that wagon-loads 

 of them could be 1 taken away from one tree. From many 

 careful experiments made on the growing seedlings, it has been 

 determined without doubt that these knees are organs of 

 respiration. All dry-land trees secure free oxygen from the soil 

 to carry on the oxidations needed in root growth. This tree 

 growing in water, which holds much less oxygen than the soil, 

 is unable to draw its full supply from the medium surrounding 

 the roots. To fill this want, these knees are pushed up out of 

 the water into the air as aerating organs. Although the plant 

 does not now grow normally on dry land, yet when planted 

 there it thrives as it once did in previous geologic ages in the 

 Arctic regions. Plant two sets of seedlings, the one in dry 

 soil, the other in soil flooded with water ; the first will show no 

 signs of the root-aerating organs, while the second will develop 

 them in abundance, as small vertical roots pushed above the 

 surface of the water. 



In the water growth, the branches of the Cypress are 

 spreading and the top flattened, a marked departure from the 

 most striking characteristic of the Pine family, to which it 

 belongs. When grown, however, in dry soil, in our parks 

 and public grounds, it reverts to the normal type. The 

 branches are short and make a sharp angle with the main 

 stem, and its cone-like form is as pointed as in any of 

 the tall conifers. This will be seen in Plate No. 2, photo- 

 graphed from a tree planted in Fairmount Park, Philadelphia. 



