584 



the depth of the water-table. Yapp ('08) arrived at a similar con- 

 clusion concerning the plants at Wicken Fen. And since the depth 

 of the water-table may influence the depth of the subterranean or- 

 gans, the latter in turn may be a potent factor in the success or 

 failure of various species. Thus, for example, the rhizomes of Polyg- 

 onum Miihicnbergii, where this species- occurs in the Sio-polyg- 

 onetum are usually at or near the surface of the stream bed. As 

 King ('97, p. 240) and others have pointed out, saturated soil like 

 that of the stream bed does not admit oxygen freely ; and so in the 

 Sio-polygonetum, the rhizomes of Polygonum and theii; roots appear 

 advantageously placed. But in the Scirpo-typhetum (PI. LXXXIX, 

 Fig. 7), where the surface soil is occupied by an extremely dense 

 mat composed of the rhizomes of Typha, Sparganiuui, and Scirpus, 

 the rhizomes of Polygonum average about 10 cm. in depth; hence 

 in the Scirpo-typhetum, although the rhizomes of Polygonum are 

 lower, evidently in response to the greater average depth of the 

 water-table, they have the additional advantage of being able to 

 tra\el with less interference from the other rhizome systems. 



An examination of Typha, SparganuDu, Scirpus Huiiatilis, and 

 S. vaUdus shows these species to be very similar in growth-form and 

 hence capable of keen competition. Where any one of these species 

 becomes more abundant in the Scirpo-typhetum, the others become 

 less so. Because of the thick, strong rhizomes, the subterranean 

 competition is to some extent mechanical; but it is probably to a 

 much greater extent, as Clements ('05, pp. 285-289) maintains, physi- 

 ological (or "physical"), especially in the case of the roots proper. 

 The opposition that any or all of these species can offer to the intru- 

 sion of other species makes their hold upon the soil very effective. 

 With Sagitfaria (PI. LXXXIX, Fig. 7), however, the case is dif- 

 ferent. Its growth- form favors a less compact arrangement of the 

 individual plants, as its rhizomes can not produce a thick mat. Ob- 

 viously, as the plants of Sagitfaria are developing vegetatively, other 

 species, such as Typha, Sparganium, and Scirpus, may easily invade 

 and occupy the soil with their densely matting rhizomes. Subse- 

 quently the rhizomes of Sagittaria, if they are to establish new plants 

 at proper distances away from the parent plant, must either plough 

 their way through the surface mat of rhizomes or travel underneath 

 it. They usually do the latter. As a rule, several rhizomes start 

 growth from each plant in early summer in a downward direction ; 

 at a depth of 10-15 cm. they assume a horizontal direction for some 

 distance ; and then grow upward again, with a tuberous, propagative 

 thickening near the distal end, and finallv resemble somewhat a shal- 



