is found in a number of species of the genus and has been de- 

 scribed by Schenck and others. Two types of aerenchyma for- 

 mation may be recognized, depending on whether the plant 

 grows in water or soil. In the latter case, the roots branch 

 irregularly and extend downward into the soil. Some or all of 

 them below the level of the ground-water are surrounded with a 

 relatively thin layer of aerenchyma, which is easily torn off by 

 pulling up the plant. The portion of the roots above the ground- 

 water level are not so covered, unless the level has recently low- 

 ered. The plant usually grows in sand close to streams, where 

 the water level is seldom more than one or two decimeters below 

 the surface. When, as sometimes occurs, it grows along small 

 muddy ditches, but with the water level beyond the reach of the 

 roots, no aerenchyma is formed. It may be found growing also 

 in shallow running water to a depth of two decimeters, and it is 

 in such places that the aerench}-ma is most richh^ developed. A 

 number of roots radiate in fairly straight lines from the base of 

 the stem, either upon or just below the surface of the soil, and 

 extend a meter or more in length. At intervals along them are 

 given off straight unbranched pneumatophores which grow verti- 

 cally toward the surface of the water. Their diameter is small, 

 about one millimeter, but they are thickly covered with aeren- 

 chyma so that the whole has a size of four to six millimeters. 

 Their growth stops when the)' touch the surface of the water, 

 but apparently may be resumed if the water rises. When the 

 water falls the tips float with the current on the surface. New 

 ones are constantly growing up and a well-grown plant may be 

 surrounded by fifty or more of various lengths and ages. There 

 are fewer of them on the roots which grow into shallower water, 

 and when, as frequently happens, the plant grows just at the 

 margin of the water the root system will show both types. The 

 necessity of some such aerating device is very apparent in stag- 

 nant water but hardly evident for running streams. 



In 1899 Dr. MacDougal delivered a lecture at Wood's Holl* 

 on the influence of inversions of temperature on the distribution 



* Biological Lectures from the Marine Biological Laboratory of ^Vood's Holl 

 1899 : 37-47- 1900. 



