126 ILLINOIS ACADEMY OF SCIENCE. 



leaf pass downward several inches below the surface and conse- 

 quently compete with the roots of both the bur-reed and the knot- 

 weed for water, oxygen, etc. The soil being rich usually in at 

 least water and nitrogenous materials, the root competition is 

 probably not serious. But before passing on we should note the 

 complementary relationship between the rhizomes of the arrow- 

 leaf and those of the bur-reed. At first it might seem that since 

 the rhizomes of both species start growth at the same level- — near 

 the siu^face — they would come to be more or less in each other's 

 way. But usually at the very outset the arrow-leaf rhizomes 

 grow downward for several inches, then take a horizontal direc- 

 tion for some distance, and finally point upward again. At the 

 distal end is produced a stem-tuber and from this arises a new 

 plant the following season. Thus the arrow-leaf may establish 

 new plants at advantageous distances from the parent plant and 

 yet, in doing so, avoid mechanical obstructions to a great extent. 

 To determine exactly how far this freedom from mechanical 

 obstruction promotes the vegetative increase of arrow-leaf plants 

 would demand, of course, accurate experimental investigation in 

 a quantitative way. Some able botanists, notably Clements, are 

 inclined against the consideration of mechanical obstruction as 

 a factor in competition ; they insist upon the importance of 

 "physical" (i. e., physiological) factors. And while in the main 

 their contentions are well founded, it cannot be denied that pro- 

 nounced exceptions do exist. For example, at Skokie Marsh, a 

 study of the water-lily association in the reed-swamp showed that 

 where the arrow-leaf was present its rhizomes had been inter- 

 cepted in great numbers by the large, semi-decayed rhizomes of 

 the water-lily {Nymphaea advena). And, in the majority of 

 such cases, the propagative stem-tubers of the arrow-leaf had 

 decayed. Even in the encasing soil, many instances were found 

 where the stem-tubers had been mechanically impeded and had 

 mostly decayed. And here, while the decay must have been due 

 to some one or more physiological causes, yet these causes could 

 not have operated had not mechanical impediments first retarded 

 the stem-tubers for a sufficient length of time. 



As our knowledge of the interrelationships of subterranean 

 organs progresses in the future, 'we shall probably find that often, 

 in the case of certain species with large subterranean parts, there 

 as ofifered or received mechanical resistance which is immediately 

 "decisive in competition because of the physiological processes that 

 it promotes. 



