Algae. 407 



which resembled the iirst mentioned form, the other the latter. Inte- 

 resting observations on the hibernating conditions are given. 



Among the Hormogoneae Aphanlzomenon and Gloiotrichia have been 

 studied most thoroughly. Aphanizomenon reaches a great maximum in 

 Vi borg so and JulsÖ and further more in Haldsö (all three lakes in 

 Jutland). Its mode of life is as follows: the spores which lie in the 

 bottom (esp. near the coasts), begin to grow in May ; the new fila- 

 ments, containing one (seldom two) heterocyst, appear in the plankton ; their 

 number at first increases slowly, but later on very quickly producing in 

 Aug.-Oct. an enormous aquatic-flower. The heterocysts become rarer at 

 the end of Aug., when the spores begin to develop; in Nov.-Dec, when 

 the spores are ripe, both they and the filaments disappear entirely. 

 The development of tufts of filaments, so often observed, is caused by 

 the filaments sticking to each other; at first they are Single, but gradually 

 become tuity by the action of the wind. Gloiotrichia was only found in 

 one lake in Jutland (Thorssö); ihe wintering and the development 

 of the globular stage have been observed. 



As regards the occurrence of the plankton forms of the 

 Cyanophyceae they reach their maxima at temperatures differing 

 for the different species, but very constant for each single 

 species. Only Chroococcus limneticus, Lyngbya bipunctata and Oscilla- 

 toria rubescens have their maxima at a low temperature (2° — 10 u C), most 

 of the others are summer-forms with maxima at about 18'— 22' C. Their 

 occurrence and conditions in relation to the plankton of alpine and low- 

 lands-lake outside of Denmark is fully discussed. 



C h a p t e r V. Diatomaceae play as prominent a part in the plankton 

 of the Danish lakes as the Cyanophyceae. The following forms have 

 been observed: *Melosira crenulata Ktz., *M. granulata (Ehbg.) Ralfs, 

 M. varians Ag., M arenaria Moore, Cyclotella comta Ehbg. var. quadri- 

 juncta, Stephanodiscus astraea ('Ehbg.; Grün , 5". Zachariasi Brun., 6". 

 Hantzschii, var. pusilla Grün., Rhizosolenia longiseta Zach., Attheya 

 Zachariasi Brun , *Fragilaria crotonensis (Edw.) Kitton, F. virescens 

 Ralfs, Synedra acus var. delicatissima Grün., 5 ulna Ehbg., * Asterionella 

 gracillima Hantzsch (incl A. formosa Hassall), Centronella Reicheltii 

 Voigt, Tabellaria fenestrata (Lyngb.) Ktz., T. flocculosa Ktz., Diatoma 

 clongatum Ktz. var. tenue and Cymatopleura elliptica (Breb.) W. Sm. The 

 4 species marked with an asterisk are of special importance. and they have 

 been exhaustively studied. The diatoms attain their maxima at a tempe- 

 rature of below 15° — 16" G.; the temperature and not the light is the 

 chief agent in determining the occurrence of the great maxima of Diatoms. 

 With regard to the many other general remarks concerning plankton 

 diatoms, the work itself must be consulted. 



Chapter VI. Chlorophyceae are of less importance in the 

 plankton of the Danish lakes especially when we compare their 

 occurrence here with the occurrence in the Swiss lakes as 

 known from the investigations of C h o d a t. The following 

 forms are recorded: Tetraspora lacnstris Lemm., Sphaerocystis 

 Schroeteri Chod., Euclorina elegans Ehbg., Dictyosphaerium pulchellnm 

 Wood., Oocystis Naegelii A. Br., 0. lacnstris Chod., Chodatella ciliata 

 (Lghm.) Lemm., Richteriella botryoides (Schmidle) Lemm., Nephrocytium 

 Agardhianum Naeg., Raphidium fasciculatum Ktz., Kirchncriclla lunaris 

 Moeb., Selenastrum bibraianum Reinsch, Crucigenia rectangularis (A. Br.) 

 Chod., C. Lauterbornii (Schmidle) Chod., Scenedesmus quadricauda (Turp.) 

 Breb., 5. bijugatus (Turp.), Ktz., Dimorphococcus lunatus A. Br., Poly- 

 cdriuiH reguläre (Ktz.) Chod., P. limneticum Borge, Pediastrum boryanum 

 (Turp.) Menegh., P. duplex Meyen, P. simplex Meyen, P. biradiatum 

 Meyen, P. Kawraiskyi Schmidle, Coelastrum microporum Naeg., C. cant- 

 bricum Archer., Hariotina reticulata Dang., Botryococcus Braunii Ktz., 

 Tribonema bombycinum (Ag.) Derb, and Sol., f. depauperata Wille, n. 

 forma., Closterium ceratiutn Perty, Cosmarium scenedesmus Delp., Stau- 

 rastrum gracile Ralts, S. paradoxum Meyen. var. longipes Nordst. The new 

 slender form of Tribonema (Conferva) bombycinum is of special interest; 



