( 692 ) 
but unknown factors may also come into play. Hence it is necessary 
to exercise the greatest caution in drawing conclusions. In advancing 
an explanation of an observed phenomenon, we must consider care¬ 
fully whether it is the only one possible and we must attempt to 
prove it in various ways by means of comparative experiments. 
The extent to which these precautions have been observed by no 
means corresponds to the complexity of the problem. As a result of 
a few experiments many observers have put forward certain expla¬ 
nations, when other explanations were equally plausible, or they have 
combated the opinions of other investigators, who perhaps had a 
more correct insight, although they were unable to adduce sufficient 
proof for it. Even serious investigators have made this mistake. I 
will illustrate this very briefly, by showing the insufficiency of the 
reasoning which led to the rejection of the possibility that the tannins 
might serve as plastic material. 
As was said above Sachs does not believe that tannins can act in 
any way as plastic material in the formation of the tissues. This 
opinion he has partly supported by observation and partly by drawing 
what is in my opinion an erroneous conclusion. Sachs x ) found that 
m the germination of seeds which do not contain tannins in the 
endosperm or in the embryo, tannins are formed in metabolism and 
primarily there, where the formation of tissue has just started. He 
never saw the tannins diminish or disappear during germination. In 
other cases, namely in that of the acorn and of the chestnut, where 
the embryo contains tannin, he did not observe a diminution either, 
but rather an increase. He made similar observations on the development 
of buds. Sachs concludes from the above-mentioned facts, that tannins 
remain in plants in the places where they have been formed, and 
that therefore they do not take part in the formation of tissues, for 
if this were the case, a diminution would have been observed. I 
consider this conclusion to be incorrect. Quite a different conclusion 
might equally well be based on Sachs’ observations, namely that the 
frequent appearance or presence of tannins in tissue-formation shows 
that these substances have probably a function to perform in this 
process. Nevertheless I do not at all consider that Sachs has proved, that 
tannins remain in the places where they are formed and that they 
do not serve as plastic material in tissue formation. For if in the 
germination of seeds more tannin is formed than is decomposed, a 
diminution o f the tannin content need not occur, and an increase 
*) Physiolog. Untersuchungen fiber die Keimung der Schminkbohne, l.c. pAlL 
Zur Keunangsg^chiehte der Dattel, 1. c. p. 246. Handbuch der Expeririiental- 
Physiologie der Pflanzen, 1865, p. 361. * . . . , 
