222 THE LIFE OF THE PLANT 



defect in the apparatus, carbonic acid should not 

 be given off in time, the bell will go on ringing without 

 stopping. I am sorry not to be able to show the 

 apparatus at work, for reasons I have already explained 

 more than once : its action depends on the giving off 

 of oxygen by the plant, and this process takes place 

 exclusively in daylight. But I can nevertheless give 

 you an idea of how it works. If we rapidly cool the 

 air under the bell (a) its volume will decrease as it 

 would have done in consequence of the absorption of 

 oxygen ; and this decrease in volume, provided that 

 the apparatus is in good working order, must be followed 

 by the same result — the giving off of carbonic acid by 

 the marble ball and the ringing of the bell. In order 

 to quickly cool the air inside the bell, I have sprinkled 

 it with some ether. The air has cooled, has decreased 

 in volume ; the column of acid has moistened the 

 marble. The marble has effervesced in giving off 

 carbonic acid, and the bell instantly rings. But the 

 effect of the momentary cooling ceases ; the usual 

 order is re-established — and the bell stops ringing. 



Thus every time the plant is threatened with a lack of 

 carbonic acid, the latter is given off by the apparatus B, 

 and this is accompanied by the ringing of the bell. If 

 the apparatus gets out of order, if carbonic acid ceases 

 to be given off — the bell rings without ceasing. 



If I had asked you a few minutes ago whether it were 

 possible to oblige the plant every time it is hungiy, every 

 time it is merely threatened by hunger, to inform us 

 of the fact by ringing a bell, you would probably have 

 considered it an untimelj- joke. And yet this is exactly 

 the significance of our apparatus. It is kept working 

 by the activity of the plant, by its power of decomposing 

 carbonic acid and giving off oxygen. We take advantage 

 of this faculty in order to oblige the plant to inform us 

 from time to time by a short ringing of the bell when 

 its feeding was going on successfully, and by beating 



