1905] PEIRCE & RANDOLPH—IRRITABILITY IN ALGAE 339 
shows, the young plants grown in the dark for five days were only 
as far along as those grown for three and three-quarters days under 
the black cap with a slit in the side. Fig. 16 shows plants five days 
old grown under the black hoods with a side slit. This difference in 
growth rate may be due to two factors at least. In the first place, no 
manufacture of non-nitrogenous food can take place in the dark, 
though some such food may be manufactured even in the dim light 
under the black cover. If such food were manufactured, it could be 
used at once to nourish the organism, which is supplied in the spore 
with only a comparatively small amount of food. In the second place, 
ES 
13 Oe 15 (fer 
13. Germinating Spores sowed 33 days earlier on ground glass in dish ing light only through 
vertical slit on one side, 17X6™™; direction of light indicated by arrow. Note that all rhizoids point away 
from light, whereas the nearly erect plantlets are inclined toward the light—14. Same in every respect, 
except that it was kept constantly in darkness. Note smaller size of plantlets. 15. Same, five days old, 
EES a terete lighted plantlets 33 days old.—16. Same as 15 but in dish lighted 
~ 48h vertical slit, 17X6™™, Plantlets stri ingly further advanced, erect but inclined toward light; 
direction of Source of light indicated by arrow. 
if the manufacture and assimilation of nitrogenous food is more rapid 
in the light than in darkness, as is now claimed,® those spores in the 
light would be better off than those in darkness. 
Whatever the reason may be, the young plants grow and develop 
faster in hormally alternating light and darkness than in continuous 
arsness. This fact, not altogether in harmony with certain current 
views of the influence of light and darkness upon growth (11), deserves 
much more extended study. It seems to us probable that the opinions 
e Plant physiologists, based too exclusively on studies of land plants, 
Will be modified when the algae, especially the marine algae, have 
we as familiar to them as aquatic animals now are to other 
Physiologists, 
ti Having Seen that light favors germination and growth, we may now 
quire what other effects it has. If spores sown in dishes in the 
6 . 
SOPLEWSKr's papers in Bull. Acad. Sciences Cracow, 1903, and earlier and later. 
