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The strength of the precipitates with antipyrine and caffeine was 
judged in various ways. Thus it was noted, whether the nucleus, 
which in normal circumstances is very clearly visible in Spirogyra 
maxima, could still be distinguished after precipitation of the tannin. 
Furthermore it was noted whether the suspensory threads, the chroma- 
tophores and the starch foci above and below in the cell could still 
be discerned. In order to judge in which cells the precipitates were 
strongest, the various cells were not only compared after precipitation, 
but it was also noted in which cells the precipitate first appeared 
and remained visible for the longest time after the filament had been 
transferred to water. I had previously found that the precipitate first 
appeared in the cells with the largest tannin content and that after 
the filaments had been placed in water, it could be observed in these 
ceils for the longest time. 
In connexion with the use which I wished to make of antipyrine- 
and caffeine solutions, it was very important to know to what extent 
these solutions are harmful to life and whether a short stay in these 
solutions, sufficiently long to obtain an idea of the tannin content, 
might be regarded as harmless or practically harmless to the Spirogyra 
filaments. I found that, if a one percent solution of antipyrine, or 
a x /m percent solution of caffeine were used, made up with ditch 
water or with distilled water (a solution of such concentration 
therefore that all or nearly all the tannin was precipitated in the 
cells) and that if the Spirogyra filaments remained in this solution, 
no further divisions took place and growth was soon arrested or 
was stopped at once. If, on the other hand, solutions were used 
which were ten times as dilute, and which did not cause a precipi¬ 
tate in the cells, it was found by comparative experiments with 
Spirogyra filaments in ditch water or in distilled water, that growth 
was retarded by antipyrine and by caffeine, and that fewer nuclear 
and cell divisions occurred. 
I made some experiments with a one percent antipyrine solution 
and with a 1 / 10 percent and a one percent solution of caffeine, in 
order to see whether a daily sojourn of 10 minutes in these solutions 
was harmful to Spirogyra, grown in ditch water. A period of 10 
minutes was selected because it is sufficient for an examination of 
the tannin content. The result of these experiments was, that it could 
not be ascertained with certainty whether the procedure employed 
was harmful to the Spirogyra. Sometimes the growth of the controls 
in ditch water was the stronger, sometimes that of the filaments 
which had been periodically treated with antipyrine and caffeine 
solutions. It is not improbable that the differences observed depended 
