256 PROFESSOR MACAIRE ON THE DIRECTION ASSUMED BY PLANTS. 
less rapid. The fumes that escape from the acid, without any actual contact with 
the liquid, cause the tendrils to contract and curl in the air, though in a less prompt 
and energetic manner than when immersed in it. The parts of the tendril that have 
been steeped in nitric acid, are afterwards found withered and dead. The tendrils, 
on the contrary, that have curled up by simple exposure to the nitrous vapours do 
not wither. 
A solution of corrosive sublimate seems slightly to excite the contraction of the 
tendrils of the Tamus ; but after a few hours the tendril so treated withers and 
dies. 
I immersed in prussic acid, prepared by Scheele’s process, a tendril of Tamus 
in its straight and vigorous state, carefully preserving it from contact with the 
vessel. I kept it there for two minutes and then took it out. The tendril did not seem 
to have experienced any alteration ; but when I placed in contact with it a foreign 
body to excite it to curl up, no contraction could be produced ; and although the 
tendril was to all appearance fresh and healthy, it could not be made to curl round. 
When a tendril that has been previously excited and has begun to curl itself in a 
spiral form is plunged into prussic acid, it loses the power of continuing its contrac- 
tion, and the knots that were commenced do not beeome tied. Two days after such 
an experiment a tendril was found precisely in the same state in which it had been 
left, although the curling had begun on a branch of proper size, and the tendril ap- 
peared endowed with complete vegetative power. I tried if I could destroy with 
ammonia this narcotic influence of prussic acid, but without any decided results. 
A fresh and healthy tendril of Tamus was excited till it began to curl, and then 
immersed for two minutes in prussic acid. The knot in its incipient state was 
placed on a branch of precisely the same size as that by which it had been excited, 
and I observed every hour what took place. During five days that the experiment 
v/as followed up, no change was produced : the knot begun did not become tighter, 
nor was any other knot formed. The tendril remained to all appearance perfectly 
healthy ; but the prussic acid seemed to have destroyed in it all power of contracting 
farther. These experiments require to be continued and multiplied ; but they 
warrant the conclusion, that whatever be the cause of the irritability of the tendrils, 
their curling-up cannot be explained, as Knight thought, by the unequal action of 
the light on both sides of the tendril ; nor as Decandolle admitted, by the obstacle 
afforded to vegetation by the contact of the leafstalk with the body on the side where 
it touches it. The rapidity of contraction in the tendrils of the Tamus cannot be 
accounted for by so slow a process. This irritability is a vital property inherent in 
the organ itself, but which ceases when it is separated from the parent plant, and 
which, as I have already shown to be the case with those plants called sensitive, 
is excited, modified, and even suspended or destroyed by the influence of poisons, 
either of the vegetable or mineral kingdom. 
