490 
F. L. PICKETT 
Variations of temperature between 13° and 25° C. affect develop- 
ment only as to rapidity. Cultures placed where they received equal 
light, but with a constant difference of ii° C. in temperature, develop 
similarly in every way, but those in the warmer place grow much more 
rapidly. This is true of plants in all culture media. Plants of cultures 
changed from a temperature of 13° C. to 24° C. show none of the peculi- 
arities described below, as long as the light conditions remain the same. 
Spores may begin germination and produce one or two rhizoids 
when sown on distilled water, but will only very rarely show any cell 
division or chlorophyll formation. One series of such cultures was 
started August 30, 1913. After being subjected to a temperature of 
16° C. and diffused light (.083) for seven weeks these cultures showed 
more than 90 per cent, of the spores with one or two rhizoids, varying 
in length from five to fifty times the greater spore diameter. A: the 
end of this period nutrient solution was added and all the spores 
with rhizoids continued normal germination. Other cultures were 
allowed to wait ten to twelve weeks before receiving nutrient solu- 
tion. Only about 50 per cent of the spores produced plants after 
the twelve week period. 
Prothallia submerged in the culture solution tend to develop long 
protonemal chains of cells (fig. 20) and but rarely produce plates. The 
growth of submerged plants is also much slower than that of floating 
plants. Plants on the surface of the culture fluid develop in every way 
as plants on soil, but do not form antheridia or archegonia. At least, 
cultures kept under favorable conditions for eight to nine months 
show no sex organs. In this discussion floating plants are considered 
excepc where otherwise stated. 
In the attempt to determine the influence of light of different 
intensities and of varying intensity, cultures with prothallia at various 
stages of development have been used. As a basis for comparison 
the history of one culture is here transcribed in detail from the record. 
The light values given were determined by means of a Solio photo- 
meter, and are based on i. = full sun i.oo P. M. of a clear day. 
Spores were sown on Knop's solution in a stender dish, October 13, 
191 3, and left in the greenhouse vestibule at an average temperature 
of 21° C. and with a maximum daily light value of .083. 
October 28, nearly all the spores have germinated and most of the 
plants are composed of three to five plump cells in chains, with 
abundant chloroplasts. Culture was placed under a saucer raised i cm. 
from the table, light value .011. 
