514 Armstrong —The Function of Hormones in 
appearance assumed by the fruit may be conditioned from within rather 
than by any environmental influence. 
As the banana ripens, the green outer skin becomes yellow and finally 
a deep brown, if not black ; the fruit is then fully ripe. The brown-black 
colour is easily produced by exposing the fruit to an anaesthetic. Moreover, 
if the interior edible part of the fruit be corked up in a test-tube together 
with an Aucuba leaf, the latter blackens sooner or later, especially if the 
temperature be raised to 35-37°. There can be little doubt that under 
natural conditions the blackening of the outer skin is due to the escape of 
an ethereal salt produced within ; a signal is thus given that the fruit is ripe. 
The autumn coloration of leaves and their fall may well, at least in 
part, be conditioned by processes of a similar order — by a sudden outburst of 
hormones which either determine the occurrence of special enzymic changes 
or hasten such changes. Similar considerations may apply to the ripening 
of seeds. 
This last autumn, when green leaves of Mangel-wurzel taken from the 
various plots at Rothamstead were exposed to the action of toluene, those 
obtained from plots which had been highly manured with nitrogenous 
materials retained their green colour more or less completely ; those from 
roots grown on other plots became yellow, resembling in this respect those 
grown under ordinary farm conditions and judged to be about ripe. 
Much discussion has taken place as to the physiological significance of 
glucosides. 1 Dunstan and Henry, 2 in summing up the position with reference 
to cyanophoric glucosides, speaking of the value of hydrogen cyanide to 
the plant, remark : ‘ At first it was regarded as merely a waste product of 
no metabolic importance ; later the view that it was possibly a means 
of protection was suggested ; and more recently a small number of botanists 
and chemists have put forward the idea that the acid is an intermediate 
product in the synthesis of proteids.’ We are inclined to take the view 
that,- in not a few instances at all events, the compound associated with 
glucose functions simply as a hormone. In some cases the cyanophoric 
glucoside disappears as the seed ripens — in Linum , for example ; maybe, 
hydrogen cyanide is of special service in hastening ripening ; or when 
present in the seed, as in mustard, the glucoside may undergo hydrolysis 
during germination and furnish a hormone which serves to stimulate the 
growth of the seedling. The extent to which hormones are producible may 
have something to do with the various degrees of readiness with which 
seeds germinate. It is well that we should point out that, from our point 
of view, it is improbable that hydrogen cyanide is ever present as such 
except in minimal amount, unless perhaps at certain special times ; it would 
necessarily escape at a rapid rate if it were thrown into circulation. 
1 Cp. E. F. Armstrong, The Simple Carbohydrates and Glucosides, p. 89. 
8 British Association Report, 1906, p. 145. 
