INFLUENCE OF EXTERNAL CONDITIONS 327 



digenetically, or monogenetically, are generally able to endure 

 much greater ranges of adverse conditions than portions of the 

 vegetative body, or any somatic product. 



The constructive process of reproduction may be carried on 

 however, within a much narrower range of trophic conditions 

 than the purely vegetative processes, the range between the max- 

 imum and minimum of any given factor being much smaller than 

 for growth. The more important factors concerning reproduc- 

 tion are the chemical composition of the substratum, composition 

 and pressure of the air, and water, oxygen, temperature, light, 

 and perhaps electric currents. A variation of any one of these 

 factors may be the cause of inhibiting the formation of spores by 

 any one method, and may set in action the mechanism by which 

 spores of a different origin are produced, or may cause a transition 

 from purely somatic propagation, to digenetic reproduction, or vice 

 versa. This may be illustrated by the following. 



427. Influence of External Conditions upon Vaucheria. If vig- 

 orous specimens Vaucheria terrestris are removed from natural 

 conditions on moist soil and cultivated in strong sugar solutions 

 no formation of spores will be shown. Similar plants kept in 

 small aquaria containing 1—3 per-cent. solutions of sugar, at room 

 temperatures, will produce sexual organs in one to two weeks. 

 If specimens are cultivated in a nutrient solution consisting of 1 00 

 cc. distilled water, .05 g. ammonium nitrate, .02 g. magnesium 

 sulphate, .02 g. magnesium phosphate, and .01 g. calcic chloride 

 for a week or ten days and then removed to distilled water and 

 placed in the dark, zoospores will be formed in a few days, per- 

 haps within twenty-four hours. 1 



'Klebs, G. Ueber einige Probleme der Physiologie der Fortpflanzung. Jena. 

 1895. 



Klebs, G. Die Bedingungen der Fortpflanzung bei einigen Algen und Pilzen. 

 Jena. 1896. 



Klebs, G. Zur Physiologie der Fortpflanzung einiger Pilzen. Jahrb. Wiss. Bot. 

 35 : Hft. 2. 1900. 



Mobius, M. Zur Lehre von der Fortpflanzung. Jena. 1897. 



Livingston, B. E. On the nature of the stimulus which causes the change of 

 form in polymorphic green algae. Bot. Gazette. 30 : 289. 1901. 



