Botany. — Light- and dark-adaptation of a plant cell. By Dr. D. 
ToLLENAAR and Prof. A. H. Braauw. 
(Communicated at the meeting of April 30, 1921). 
Now that it has appeared that the growth in length of vegetative 
organs as a rule shows a very characteristic response to the light- 
stimulus, we possess in this response of growth to light an excellent 
criterion for the elementary study of sensitiveness to light. In this 
research of which all experiments have been made by Mr. TOLLENAAR, 
we have carried on the study about the way in which the light 
sensitiveness of an individual cell, the sporangiophore of Phycomyces 
nutens, reveals itself in the response of growth. The progress of the 
reactions is already known in case that fixed quantities of light of 
"/, M.C.S. to 2 mill. M.C.S. are applied in a short time to cells that 
are kept in the dark (see Licht u. Wachstum I). Likewise how the 
response of growth is, when the cell already adapted to the dark 
is exposed to permanent light. e.g. of 1,64 or 4000 M.C. (see Licht 
u. Wachstuim III). In 64 M.C. for instance we see acceleration- and 
retardation of growth interchange and gradually brought into equi- 
librium. When after this adaptation to light the growth has become 
constant again, its progress appears in 64 M.C. some percentages 
quicker than in the case of the cell adapted to the dark. 
I. Conversely, does the growth become some percentages slighter, 
when the cellafter adaptation to light has been completely re-adapted 
to the dark in 14 or 2 hours? 
All experiments in this research have been made at 16° C. with 
horizontal and 4-sided illumination. For the cultures + stems 
were used. 
Experiments of control. These are required, because the growth 
of the cells may somewhat change its speed in the course of 2 hours, 
even though the illumination remains constant, in consequence of 
chance causes or e.g. because the growth is still increasing or already 
decreasing. With cells that have been adapted to 64 M.C. the rate 
of growth differed after two hours, respect.: —1, + 3}, + 1,—1, 
— 74, + 74, — 24, — 14, —7, —5, +3, 0, +1, + 14, + 4°/,. 
average —0,27°/,, i.o.w. after adaptation to 64 M.C. (in 2 hours 
the average growth remained the same in the next 2 hours in 64 M.C 
2 
Proceedings Royal Acad. Amsterdam. Vol. XXIV. 
