406 Algae. 



arrived at, may be true to nature". Dr. W.-L. iinishes the introduction 

 with some remarks on the deiinition oi the species; plankton-investigators 

 have chosen one oi two different methods : 1. Separation oi a consi- 

 derable number of species, 2. unition oi many species together; he ad- 

 mits that he comes nearer to the latter method than to the former, re- 

 ferring to the wellknown fact," that temporal as well as local Variation 

 in regard to various plankton-organisms is, so speaking, conditio sine 

 qua non oi their occurrence in the pelagic region." 



Chapter I contains a topographical description oi the districts 

 examined and gives an account oi the size and depths oi the lakes, their 

 elevation above sea level, their outlets and aiiluents. The 8 maps are 

 illustrations to this chapter. 



Chapter II. The method oi research: plankton me- 

 thod ic. The samples from the 9 lakes have been collected simul- 

 taneously and consequently by different collectors; the author himself has 

 collected the samples of the Zealand lakes. The nets used were 7 in 

 number with gauze of various fineness; for collecting of the phyto- 

 plankton the millergauze No. 20 was used, and the samples were pre- 

 served in formalin. These regulär collections give a good knowledge as to 

 which organisms occur in the lakes in question and to the time for their 

 occurrence, and they throw light upon the factors that play prominent 

 parts in the occurrence of the great maxima ; they have proved, that the 

 maxima and minima of the Single species occur somewhat simultaneously 

 in all the lakes examined and that these phenomenons with regard to time 

 coincide exactely with those oi other countries. This similarity according 

 to the author, is a security for the exactness of the records oi irequency 

 (ccc, cc, c, -\-, r, rr) answering fairly well to the real conditions. Ne- 

 vertheless there is no doubt that the marks of frequency have not the 

 same value with the different authors, and Dr. W.-L. has therefore taken 

 microphotographs of 12l) samples to give other investigators a somewhat 

 reliable idea of what he understands by the marks. These microphoto- 

 graphs reproduced in 10 plates are very instructive and give a good idea 

 of the varying associations of plankton in the different lakes and in the 

 same lake at different seasons; they will without doubt be succeeded by 

 analoguous microphotographs from other countries. 



Chapter III. Some primary notes on the bottom depo- 

 sits and physical conditions of the lakes, and especially 

 of their temperature. This chapter is mostly of geological and 

 hydrographical interest. The temperature-investigations are the most 

 interesting; all the Danish lakes are shallow, but there is a marked diffe- 

 rence between a more shallow group and another which is somewhat 

 deeper (20 — 30 metres), the first group was frozen at two diiferent periods, 

 the second only once; Haldsö in Jutland differs most from the type. 

 For further details the work itself must be consulted. 



In the following chapters the different groups of plankton-organisms 

 are treated by enumerating all the species observed and giving very 

 exhaustive remarks with regard to their occurrence and frequency, further 

 by interesting observations on the development of many of the species, 

 constantly taking the records of other plankton-investigations into con- 

 sideration. 



Chapter IV. Cyanophyceae. The blue-green algae play a great 

 part in the plankton of the Danish lakes; they often produce „aquatic- 

 flower". Chroococcus limneticus Lemm., C. minutus Näg., Merismo- 

 pedium elegans A. Br , Gompliosphaeria lacustris Chodat, Coelosph aerium 

 Kützingianutn Näg., Polycystis aeruginosa Ktz. (incl. P. flos aquae Wittr.), 

 Lyngbya Umnetica Lemm., L. bipunctata Lemm., L. contorta Lemm., 

 Oscillatorla rubescens DC, Anabaena flos aquae (Lyngb.) Breb., A. spi- 

 roides Klebahn, A. macrospora Klebahn (Syn. A. stricta Ostf.), Aphani- 

 tomenon flos aquae Allrn. and Gloiotrichia echinulata (Engl. Bot.) P. Richter 

 are recorded as limnetic forms. Polycystis aeruginosa and P. flos aquae 

 are united, because the two forms are connected by numerous transi- 

 tiorial stages; colonies have been observed (in Viborgsö), one half of 



