IX THE POLYPODIACEAE 381 



was associated with conditions of the environment. In Klebs's 

 experiment weak light seemed to have been the cause of branching. 

 Light may be the determining factor in branching where the pro- 

 thallia are crowded and therefore shaded by other prothallia, but 

 it can not be considered the determining factor in all the cases 

 described in this paper, for the majority of these branched pro- 

 thallia occurred in uncrowded regions of the cultures under good 

 light conditions. However, all the branched prothallia except 

 one developed on cultures of distilled water or nutrient solutions, 

 from which some chemical element had been omitted; while all 

 but two of the branched prothallia on the soil cultures were found 

 on the inner surface of the pot wall. This is especially interesting 

 as indicating that other factors than light entered into the problem. 

 Atkinson designates the branched prothallia of Adiantum as 

 "starved prothallia," which would seem to indicate that they had 

 developed under poor nutritive conditions. Miss Black suggested 

 the possibility of a lack of oxygen as the determining factor in 

 the production of the branched prothallia in her cultures. This 

 explanation would not hold good for the present cases, as all three 

 cultures, distilled water, soil, and nutrient solution, were covered 

 with loosely fitting glass plates and, since the prothallia were not 

 submerged in the nutrient solution or distilled water they received 

 a sufficient supply of oxygen. 



xA.s branching was not observed in the nutrient solution cultures, 

 where all the chemical elements necessary for growth were present 

 in sufficient quantities to meet the needs of the prothallia; as few 

 cases of branching were observed among the prothallia of the 

 soil cultures, in positions which appeared favorable for nutrition, 

 it would seem as if there was an intimate connection between 

 nutrition and branching, that poor nutritive conditions accelerated 

 the stimulus of branching, and that good nutritive conditions weak- 

 ened the stimulus. However, it is impossible to say just what factor 

 of nutrition was the determining one; for even by cultivating pro- 

 thallia on nutrient solutions whose chemical formula are known, 

 we do not know the physiological effect from the chemical reactions 

 due to the presence or absence of various chemical elements. 



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