Floristik, Geographie, Systematik etc. 47 



samaramirri, P. nigrmn f. glahrispica, P. arborisedenSj and P. Mer- 

 ritti parvifolium. Trelease, 



North American Flora. Volume IX. (Published by the New 

 York Botanical Garden, Bronx Park, New York City. 1907—1916.) 



With part 7, consisting of indexes, title-page and contents, 

 this, the first volume to reach completion, is finished, It deals with 

 part of the Agaricales, and is largely the work of W. A. Murrill. 

 The several contributions to the volume have been noted, as issued, 

 under the names Barnhart, Burlingham, Murrill, and Pen- 

 nington. . Trelease. 



Fallis, M., The Structure and History of Plav: the floa- 

 ting Fen of the Delta of the Danube. (Journ. Linn. Soc. — 

 Botany. XLIII. p. 233—290. 15 pl. and 1 fig. 1916.) 



The authoress has already published observations on the Vege- 

 tation and topography of English Fen (Norfolk Broads), and the 

 investigations here described are an extension and a comparison 

 with conditions as presented on a wide and almost primitive scale. 

 The topographical description of the Danube and the Balta 

 (or inundation district of the Danube in Rumania) is illustrated 

 by an excellent map reproduced from Gr. Antipa's earlier work 

 (1912). The influence of the seasonal floods on the water-level of 

 the Balta is shown to be an important factor in the evolution of 

 Plav. The preliminary description by Antipa on the structure and 

 origin of Plav is summarized and amended according to the obser- 

 vations of the authoress. The plant which plays the most important 

 part in the evolution of Plav is Phragmites communis^ Trin., var. 

 ßavescens, Gren. & Godr. Plav consists mainly of submerged ver- 

 tical rhizomes of this species, bound together by water- roots which 

 retain much soil, and forming floating rafts of considerable thick- 

 ness (1 — 2 metres). These bear the aerial vegetative and flowering 

 shoots of Phragmites (Reed), and a number of other species, which 

 by their death and decay give rise to a surface soil mainly organic. 

 The submerged part of te Plav was investigated by sections, and 

 its structure is illustrated b}'^ a series of excellent photographs. 



The Reed {Phragmites) shows three well-marked phases of 

 growth: a) open reed-swamp with a sparse growth of reed-shoots 

 advancing into open water; b) closed reed-swamp where concentric 

 growth has ceased; c) Plav or masses of reed detached by storm 

 or changes in water level. Factors essential for the detachment of 

 Plav are death of the basal rhizomes, a sufficient depth of water 

 and a limited deposition of silt by flood-water. In connection with 

 the growth-cycle of Phragmites, there is developed an important 

 thesis on „what constitutes an individual", and the views advanced 

 have an important bearing throughout all phytogeography. In the 

 case of Phragmites on the Plav there is a marked Variation in size 

 between the „giant reed" (up to 5,15 metres) and „slender reed" (2 

 m.). These are not regarded as floristic varieties but as phases in 

 one life-cycle, the giant shoots in youth, the smaller ones in old 

 age. Two plant units are postulated: a major unit and a minor unit. 

 The major is the total vegetative Output initiated by one fertilised 

 cell, it is a constant with an absolute age, and its mass is the mea- 

 sure of specific vital energy. The minor unit, produced vegetati- 

 vely, is each reed-shoot. In Phragmites the giant shoots are the 



