2i8 THE LIFE OF THE PLANT 



But we must now return to our cress and asparagus. 

 You may have noticed, while I have been talking, how 

 the patch of light has been steadily creeping up the 

 wall ; it is no longer at the tenth division, but some- 

 where near the fortieth. This is because the mirror has 

 passed from the position mn to that of rs ; so we have 

 seen for ourselves how the plant grows. At the same 

 time the root-tip of the cress has long ago moved 

 beyond the circle traced round it with the pencil, and 

 has considerably elongated. This means that we can 

 observe not only the result of this process, not only its 

 immediate cause, i.e. the growth and division of cells, 

 but that we can also grasp the very process, i.e. the very 

 movement that we call growth.^ 



We have thus performed the first part of the task 

 we set ourselves at the beginning of this lecture. But 

 what of the second part ? Can we hear how the plant 

 vegetates? Can we, for instance, make the plant tell 

 us by means of sounds of some kind how it thrives ; 

 whether it is hungry or satisfied ? The following ex- 

 periment will show us that we can. A plant is being 

 grown in an artificial soil ^ under a glass bell, with 

 its flange carefully ground (fig. 64, A). We know, 

 however, that one of the most important sources of 

 a plant's life is the carbonic acid of the atmosphere. 

 How can we guarantee a continual source of car- 



* In order to ascertain how much the plant has really grown, we have 

 only to measure the distance between the axis of the pulley and the wall. 

 The real increase in growth, as has been already said, will be as many 

 times smaller than the visible transposition of the image of light as is this 

 distance greater than half the diameter of the block. Evidently, if people 

 at the back of this large audience can be shown with the help of this 

 apparatus the growth of a stem during the interval of an hour or even 

 half an hour, the observer standing nearer wUl be able to notice the 

 displacement of the beam of light during the interval of one minute. In 

 fact this method permits of our observing, minute by minute, the increase 

 in length of the stem ; and of seeing the movement, so to speak, as if it 

 were the movement of a minute hand on a clock. 



• See chapter iv. 



