11 
daylight go back through plasmolysis already half an hour after the 
commencement of the stimulation. It follows then that here a 
modification in the nature of the cell-wall is answerable for the 
more or less considerable growth, which fact is quite in keeping with 
the explanation of the curvature of unicellular organs. 
Braauw Le. demonstrated in Phycomyces that photogrowth-reaction 
reveals itself only when the growing topzone of about 3 or 4 mm., 
is lighted. From this also he concludes that the light does not act 
through change of turgor. 
The question now arises whether that change of cell-wall is primary 
or secondary, in other words: is the sensitive system to be found 
in the cell-wall or in the protoplasm. The latter is the more likely 
supposition, but it should be borne in mind that an investigation of 
R. HANSTEEN Cranner') lately informed us that the cellwall of the 
living cells is much more complicate than had formerly been suspected, 
viz. a complex colloid system which also contains lipoids. The whole 
problem of the growth of the cellwall, formerly interpreted through 
Opposition and intussusception, will now have to be looked upon 
from a colloidochemical point of view. 
More evidence has been produced, however, for the assumption 
that the sensitive system lies in the protoplasm, perhaps in the 
stable boundary layer. Drwyer and Hanssen’s’) research showed that 
proteins coagulate under the action of rays of light of short wave- 
length. This action is a reversible process, as has been seen also 
heretofore for growth-reaction. 
Be this as it may, in either case it is the chemistry of colloids 
which has to deepen our knowledge of the photo-chemical phenomena 
governing the photogrowth-reaction. 
1) R. HANSTEEN CRanner, Jahrb. f. Wiss. Botanik 1914. 
*) G. DREYER and O. Hanssen, Compt. Rend. T. 145, 1907. 
Cf. also F. ScHanz Ber. d. d. Bot. Ges. 1918. 
