586 



Species that are plainly complementary in one association may be 

 less so in another. Thus, Polygoninn Miihlenbcrgii and Sparganium 

 are complementary in the Scirpo-typhetum ; but in the Sio-polygone- 

 tum, where their rhizomes lie in common near or at the surface of 

 the stream bed, they are "edaphically" (see Woodhead, '06) com- 

 petitive, and hence complementary only in an aerial way. In this 

 particular case, however, the frequently open appearance of the vege- 

 tation in the Sio-polygonetum indicates that the mutual biotic strug- 

 gle of the two species is less keen than their separate struggles against 

 somewhat adverse environmental conditions. 



In the reed swamp certain mints become conspicuous during mid- 

 summer, particularly so in the Scirpo-typhetum, where they thrive m 

 the shelter of Typha and other tall plants. Teucrium occidentale and 

 Scutellaria galcricnlata are very common. They produce from their 

 basal nodes numerous slender stolons that run out at different depths 

 in the soil, and these stolons may produce new plants. These species 

 tend to have their root systems 3-6 cm. lower in wet situations than 

 in dry, although exceptions to this rule are not rare. But whether 

 growing from plants in dry or from those in wet situations, the new 

 stolons exhibit a remarkable power of changing their direction of 

 growth, in response to numerous obstructions, and thus they may 

 proceed further without serious results. Considering the strength 

 and size of the rhizomes of Typha, Sparganium, and Scirpus, also 

 the delicate nature of the stolons of Teucrium and Scutellaria and 

 their capacity for altering growth-direction, it is probable that me- 

 chanical competition between such rhizomes as those of Typha and 

 such stolons as those of Teucrium is practically absent. Again, the 

 aerial parts of the Typha form vegetate chiefly in higher atmospheric 

 strata than do those of the Teucrium form. Evaporation readings 

 show that in higher strata evaporation is much greater; and while 

 plants of relatively xerophytic structure (e. g., Typha, Sparganium, 

 and Scirpus) are fitted to withstand acute drying conditions, plants 

 with foliage of looser texture (e. g. Teucrium and Scutellaria) can 

 vegetate better in lower strata, where the effect is that of greater 

 humidity, the abundance of the latter plants among the former at 

 Skokie Marsh tending to confirm this statement. Further, the per- 

 sistence with which tall plants like Typha become dominant under 

 favorable soil conditions shows that they are not, at least not notice- 

 ably, harmed by plants like Teucrium. If, finally, we allow for the 

 great availability of nitrogenous foods in the soil and for the dif- 

 ferences in food requirements, it becomes clear that the numerous 



