ECOLOGICAL ADAPTATIONS OF CERTAIN FERN PROTHALLIA 493 
adding Knop's solution. The resulting growth was very uniform in 
rate (fig. 24), and most of the plants showed the first division indicative 
of plate formation at the same time. But when such a culture was 
removed from full to reduced light, the results were by no means 
uniform. The larger number of plants show the formation of atten- 
uated cells already described. But side by side with these forms are 
other plants showing no tendency toward such growth. Again the 
same situation is found in cultures subjected to two to four alternating 
ghanges as in fig. 31 a-h. Here some plants are shown continuing 
regular growth and plate formation without any sign of branching or 
attenuation. The growth, measured by the number of cells formed, 
is not quite as rapid under conditions of reduced light as in full light. 
It must be noted, however, that in plants grown from the first in 
light just too weak (.011) for normal development this difference does 
not appear (fig. 30). The sensitiveness to light variations is also 
shown by the following data. Cultures were grown continuously in light 
of maximum value of .025 and produced plants six to ten cells in length, 
slightly attenuated. From the middle of December up to the latter 
part of February there were alternating periods of three to ten days 
of very bright and very dark weather. During this time the cultures 
mentioned above showed the zones of attenuated cells and plates, 
just as produced by the more extreme changes of the experiments 
(fig. 27). This at least suggests that the changes in growth are due 
to the variation in light intensity and not to its absolute value. This 
view is further substantiated by the fact that in a few cases plants 
grown in light of maximum value of .025 and then brought into light of 
.8 maximum value produced the outgrowths regularly produced after a 
change from strong light to weaker light. It seems from this that the 
variation in light intensity is only a stimulus to a change in growth, and 
does not determine the nature of that change. Finally it must be 
noted that as a result of changes of light intensity some plants tend 
to branch rather than produce attenuated cells. Extreme cases are 
much like that in fig. 20. A younger and more moderate case is shown 
in fig. 28. Such plants finally produce several apical groups and 
plates (fig. 34). 
As has already been suggested there is much difference in the origin 
of the branches and attenuations which follow the change in light 
intensity. In a small number of cases the apical cell produces the 
outgrowth '(figs. 25, 26a, 32). But more often such growth does not 
