IN 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 Adzantum 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. 
As 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, itis 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. 
