1912] SHERFF—SKOKIE MARSH 423 
Scirpus, the rhizomes of Polygonum average about to 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 gain the additional advantage of being able to 
travel with less interference from the other rhizome systems. 
An examination of Typha, Sparganium, Scirpus fluviatilis, and 
S. validus 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, as CLEMENTS 
(3, pp. 285-289) maintains, it is probably to a much greater extent 
physiological (or ‘“physical”), especially in the case of the roots 
proper. The opposition that any or all of these species can offer 
to the intrusion of other species makes their hold upon the soil 
very effective. With Sagittaria (fig. 4), however, the case is 
different. Its growth-form favors a less compact arrangement of 
the individual plants. Its rhizomes cannot produce a thick mat. 
Obviously, as the plants of Sagittaria are developing vegetatively, 
other species, such as Typha, Sparganium, and Scirpus, may easily 
invade and occupy the soil with their densely matting rhizomes. 
Subsequently the rhizomes of Sagittaria, if they are to establish 
new plants at proper distances away from the parent plant, must 
either plough their way, along through the surface mat of rhizomes 
or travel underneath. 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 1o-15 cm. they assume a 
horizontal direction for some distance; they then grow upward again, 
with a tuberous, propagative thickening near the distal end, and 
finally resemble somewhat a shallow, inverted arch.?. Thus inter- 
ference from surface rhizomes and roots is to a great extent avoided. 
In this case, then, while it is not certain that the inverted arch of 
the Sagittaria rhizome is a direct adaptation to this particular 
struggle, it is certain that the inverted arch, however induced 
originally, is here of the greatest value. 
7 For illustrations of the similar rhizomes of Sagitlaria sagittifolia see GitcK 
(6, pl. 6 and figs. 35 and 39). 
