588 



Scirpus, or Ranunculus and MyriopIiyUuni and Radicula, or Polyg- 

 onuin and Veronica, or vSi/f/;! and Ruine.v are mutually competitive, 

 yet a complete community (as shown, e. g., in PI. XC, Fig. 9) is 

 complementary ; the basal parts chiefly because of different depths,' 

 and the upper parts chiefly because of different growth- forms. 



An inspection of the Nymphaeetum shows that only where Nyni- 

 phaea is nearly or quite absent does Sagittaria latifolia successfully 

 invade from the Scirpo-typhetum. As is commonly known, the rhi- 

 zomes of Nyniphaca in many habitats are usually decayed to within 

 a short distance of the growing apex. An investigation during Au- 

 gust, 191 1, showed that generally where the rhizomes of Sagittarm 

 had penetrated these decayed parts, they themselves had started to 

 decay.* Frequent cases were found where the decayed Nyiiiphaea 

 rhizomes lay nearer the surface and the Sagittaria rhizomes had pro- 

 ceeded underneath, unharmed. In many instances, however, where 

 the stem-tubers had been mechanically impeded (by woody roots, etc.) 

 in the encasing soil, they had 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 impedi- 

 ments first retarded the stem-tubers for a sufticient length of time. 

 As our knowledge of the interrelationships of subterranean organs 

 progresses in the future, we shall proiiably find that often, in the case 

 of certain species with large subterranean parts, there is offered or 

 received mechanical resistance which is immediately decisive in com- 

 petition because of the physiological processes that it promotes. 



Speaking in a general way, while Nyniphaca- and Sagittaria thrive 

 better in the Nymphaeetum and Scirpo-typhetum, respectively, yet 

 along the line of tension between these two associations the injury 

 done by the decayed Nyniphaca rhizomes to the rhizomes of Sagit- 

 taria is a factor that appears to be decisively in favor of NynipJiaca. 

 The inverted rhizome arch of Sagittaria, useful in the Scirpo-typhe- 

 tum, is here more often harmful. 



In many parts of the Irido-acoretum, Polygonum Muhlcubcrgii 

 and Galium Claytoni abound, and tliese form with Acorns a comple- 

 mentary community (PI. XC, Fig. 10). The creeping stems of 

 Galium root upon the soil surface, the rhizomes of Acorus lie just 

 beneath, and those of Polygonum are deepest of all. The bushy 

 rhoot of Galium appears not to harm the slender, ensiform leaves of 

 Acorus, and they in turn do little harm to it. In late summer, the 



*Many litmus tests uniformly showed the decayed parts of the Nyiiipliaea rhi- 

 zomes to be strongly acid. Enough cultural experiments have not been performed, 

 however, to determine whether the effect upon tlie Sagittaria rhizomes, as above 

 noted, was due to acid or to other causes. 



