354 Annals of the Carnegie Museum. 



The Potamogeton Formation. 



This formation has undergone no change from its structure in the 

 earher stages except that Potamogeton pectinatus is more abundant and 

 is more frequently found fruiting than was the case in the earlier 

 stages. No rosettes are to be seen in shallower water, 'as in Lagoon 

 Aa, as this area has been taken over by the Typha-Scirpi/s formation. 



The Typha-Scirpus Formation. 



The structure of the formation is, typically : 



Fades. — 



Scirpiis Ai?iericamis, Scirpus validus, 



Typha latifolia. 

 Principal Species. — 



Eleocharis olivacea, Sparganium eurycarpum. 



Secondary Species. — 



/uncus balticus littoralis, Utricularia coi'nuta, 



Eleocharis obtusa, Nostoc sp. 



This formation has its outer limit in about one and one-half feet of 

 water and from here it extends back on the bank to a height of about 

 eight inches above the water level. The habitat might thus be said to be 

 amphibious, but to the plants it is uniform in that the top of the stem is 

 in the air and the roots are in a saturated soil. Thtjuncus-Eieocharis 

 formation probably added some humus and thus increased the capil- 

 lary power of the soil. The soil is here made dark with moisture 

 back to a height of about a foot above the water-level of the lagoon. 

 The lower part of Xht Juncus-Eleocharis formation is supplanted by 

 the Typha-Scirpus formation ; in places almost the entire habitat has 

 been so occupied. 



The Typha latifolia consocies appears in the deeper, submerged 

 portion of the habitat, and alternates with the Scirpus validus consocies. 

 The Scirpus americanus consocies occupies by far the most important 

 place in the formation, but it exhibits more or less zonation with the 

 Scirpus validus consocies, which usually occupies the outer, more 

 deeply submerged zone. Excepting Sparganiutn, which occurs in the 

 Typha latifolia consocies, the principal and secondary species of the 

 formation occupy the bank in the Scirpus americanus consocies. In 

 places the soil around the bases of the stems of the Scirpus is almost 

 completely covered with little pellets of a species of Nostoc. 



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