31 
especially Arisz (1915), Bremekamp (1918), v. Dn. SANDE BAKHUYZEN 
(1920),| occurring in consequence of longer exposures must not be 
exclusively taken as a consequence of the progress of the responses 
of growth. 
It is perfectly true that part of the phototropical “tone”-phenomena 
may be solved as a result of the process of growth at the front- 
and backside (see v. D. SANDE BAKHUYZEN). But it should not be 
forgotten, that there is also a real change of tone which has a 
deeper base and is much more important in principle. In prolonged 
exposure real changes of tone (= phenomena of adaptation, i.e. 
changes in the degree of sensitiveness to light) principally act a very 
important part in part of those processes, which underlie the grow th 
and precede the result of the growth. The result of this altered 
sensitiveness appears to us in the response of growth as an external 
symptom. We had better not use the word sensitiveness of growth 
(as we previously did occasionally, see L. u. W. III), because it 
might be easily forgotten, that at bottom sensitiveness to light rests 
with part of those deeper processes of metabolism, from which the 
growth secondarily results. Yet for convenience’ sake we may call 
the growth (as well as the whole plant) “sensitive to light” as it 
is generally done in physiology with a number of phenomena of 
sensitiveness ; provided we remember, that in most cases but some 
primary fraction of the larger complex is really sensitive to that 
factor and perceives it, while all the rest of the phenomena are 
only resulting reactions. 
We have had to restrict ourselves to a summary of our research. 
The full data and a further discussion of the results and the 
literature will be published later on, while the researches on responses 
of growth are being continued. 
Wageningen, March 1921. 
LITERATURE. 
Arisz, W. H., Untersuchungen ü. d. Phototropismus. Rec. d. Trav. Bot Neerl. 
195712: 
Braauw, A. H., Die Perzeption des Lichtes. Rec. d. Trav. bot. Néerl. 1909. 5. — 
Licht und Wachstum | (Zeitschr. f. Bot. 1914. 6), Il (Zeitschr. f. Bot. 1915. 7), 
Ill (Meded. v. d. Landbouwhoog. 1918. 15). 
BREMEKAMP, ©. E. B., Theorie des Phototropismus. Rec. d. Trav. Bot. Neerl. 
XV 1918. 
GriinperG, K., Unt. ti. d. Period. d. Nachbilder. Z.schr. f. Biologie. 1913. 61. 
Piper, H., Ueber Dunkeladaptation. Z.schr. f. Ps. u. Ph. der Sinn. 1908. 13. 
