250 SCIEXCE PROGRESS. 



will often bring plants to flower which refuse to flower 

 otherwise, or only to a less extent. 



Over and over again attempts have been made to cor- 

 relate the conditions of the environment and the habit 

 of the plants subject to it, some of the most recent 

 being Warming {CEkologische Pflanzengcographie, 1896), 

 Gaston Bonnier [Inflzience de la Lumiere eledrique sur la 

 forme, etc.. Rev. Gen. de Bot, 1895) and Moebius iyBeitr'dge 

 zur Lehre von der Fortpflauzwig, 1897), but the critical 

 reader always feels that even the most careful experimenter 

 is unable to solve such problems, for two chief reasons : 

 (i) The (higher) plants experimented upon are so exceed- 

 ingly complex that it is almost impossible to disentangle 

 the reactions to the conditions imposed by the experimenter, 

 from correlated internal changes, and (2) it seems as yet 

 impossible to vary one factor of the environment without 

 at the same time causing others to vary also. For instance, 

 in experiments with higher plants, we cannot modify the 

 intensity or quality of the light by means of screens, etc., 

 without at the same time altering the tenipej^atiire of the 

 soil, plant or air : if we vary the temperature, then changes- 

 in moisture are induced, and so on. 



Now Klebs has chosen subjects and methods which 

 reduce these difficulties to a minimum. By selecting Algae, 

 which grow at low temperatures and in water, and by con- 

 fining his attention to the conditions which affect repro- 

 duction, he is able to gro nearer to the ideal variation of 

 one factor at a time than most experimenters have done. 



The princpal Algae employed are — Vaiickeria, Hydro- 

 dictyon, Botrydiiim and Protosiphon, Spirogyra, Desmids, 

 QLdogoniuvi, Ulothrix, Horr,Adi2tm, Conferva, Bimtil- 

 leria, Stigeocloninui, Draparnaldia, Chlamydomonas, and 

 Hy drums. 



A brief outline of his programme with Wuicheria will 

 serve as an index to his choice of conditions. He first 

 investigates the effects of nutrition, moisture, light, tem- 

 perature, inorganic salts, organic compounds, osmosis, 

 acids and alkalies, the partial pressure of oxygen and the 

 effects of moving water, on the development of the asexual 



