494 
F. L. PICKETT 
take place from the apical cell or any of the neighboring cells, but from 
older cells in regions of reduced activity (figs. i6, 17, 18, 19, 21, 22, 
26h, 29). When plants with plates 1-2 mm. wide are subjected to 
reduced light the branches or attenuated cells are produced by any 
part of the margin, from the original protonemal cells up to the apical 
group, and even at times by included cells growing out perpendicular 
to the surface of the plate. In other cases the whole apical group is 
pushed out by a band of slightly elongated cells until a distinct pluri- 
cellular branch is formed (fig. 35). To make sure that these results 
were not unduly influenced by the fact that the plants were grown on 
culture fluids, regular soil cultures were subjected to the same condi- 
tions. The results were the same with the exception that, in the case 
of plants with well developed plates .5 mm. or more in width, the 
production of broad outgrowths like fig. 35 was rather more usual 
than with the plants of water cultures. 
Summary of Reaction to Light. — The results of experimental work 
relative to reaction to light stimulus may be summed up as follows: 
Plants develop normally in almost identical manner under approxi- 
mately constant light intensities within a wide range. 
Most of the plants respond to a sharp reduction of light by produc- 
ing elongated cells or pluricellular branches. 
Some plants show a similar reaction when the light intensity is 
increased. 
Some plants do not respond to variations in light intensity by any 
peculiarity of growth. 
It seems that the change in light intensity stimulates unusual 
growth in most of the plants, but the plants show a marked individual 
variation in their response to that stimulus. 
Field Work. — ^The extreme conditions of the summer of 191 3 have 
already been given. To show that the Asplenium platyneuron fields 
under observation received the full force of these conditions it need 
only be said that many clumps of mature sporophytes were killed 
outright, after maturing a few spores in the early summer. On 
December 3, 19 13, a careful search was made for prothallia in the field. 
Numerous well developed plants were found on the soil, limestone 
fragments and dead twigs or leaves about the old sporophyte clumps. 
Most of these ranged from ten to fifteen cells up to those maturing 
their first antheridia, and averaged a little smaller than the plants of 
greenhouse cultures from spores sown September 4, 1913. A second 
