32 
F. F. Blackman. 
of the darkened leaf may be found to have gone up to the original 
level, to fall off quickly again on further starving. 
It is of great additional interest to note that however much 
assimilatory sugar is thus given, the respiration is not brought 
above the original level, but is only enabled to persist the longer 
at that level. This we interpret as meaning that the respiration of 
a fully nourished leaf is limited by the amount or mass of the 
respiratory enzyme or “ enzyme branches of the protoplasm ” 
present in the cells, the sugar being in excess. Here we have a 
first clue to the nature of the biological regulation of a single 
function in the plant-cell: one can conceive that were it desirable, 
for purposes of growth, for example, to increase the energy¬ 
providing oxidation, then the mass of enzyme present would be 
added to. In other cases and for other reasons it might be 
required to reduce the amount of enzyme. The consideration of 
how new enzyme particles arc formed is quite above the level of 
our present knowledge, but we have seen how they may be 
destroyed in discussing Albert’s work on the ultimate hydrolysis by 
a proteolytic enzyme of the zymase of etherised yeast-cells. 
That such changes go on in living cells has been admirably 
shown by Czapek' for one very interesting case. In the cells of 
the roots of Vicia, homogentisinic acid is continually being formed 
from tyrosin by the action of an enzyme, but the acid so formed 
does not accumulate, but is, in its turn, destroyed by an oxydase. 
The amount of homogentisinic acid present is thus the balance 
between two opposed reactions. It fortunately happens that the 
amount of this acid can be accurately determined by titration of 
the extract of the roots by means of silver nitrate. 
Czapek found that the mere fact of placing a root horizontal 
instead of vertical brought about an increase in the amount of 
homogentisinic acid present. This he attributes to the formation 
of an “ anti-enzyme ” which stops the action of the oxydase, whose 
function it is to destroy the acid. This effect seems to be a fairly 
direct result of the geotropic stimulus resulting from the horizontal 
position, since it comes on before curvature, (being at its height 
in 30 minutes and then passing away), and it is produced even on 
the klinostat, where no actual curvature takes place because the 
stimuli are continually counteracting one another. 
This is the first case of a direct demonstration that the rate 
of enzyme activity in the cell is in close correlation with external 
' Annals of Botany, XIX., Jan., 1905. 
