200 THE LIFE OF THE PLANT 



little hook at the end of the silk thread is fixed as 

 before to the plant, and naturally every increase in 

 growth which results in even the minutest rotation 

 of the pulley, and the mirror connected with it, will 

 cause a considerable displacement of the spot of light. 

 If the index magnified the amount of growth a hundred 

 times, the apparatus with the mirror will magnify it 

 many thousand times, or, generally speaking, any de- 

 sired number of times, since the magnification only 

 depends on the distance of the mirror from the wall. 

 In order to be able to judge more conveniently, as 

 well as more exactly, of the displacement of the patch 

 of light, large divisions are marked on the wall. Let us 

 notice the mark at which it stands at present — it is 

 exactly on the figure lo — and let us leave the plant 

 (which happens to be a shoot of asparagus) to grow in 

 peace, and then return to it at the end of our lecture. 



Possessing such perfect means for observing growth, 

 botanists have been able to study a whole range of 

 questions concerning these phenomena. Thus for 

 instance, the reason for the above-mentioned fluctua- 

 tion in the rate of growth with night and day has been 

 investigated. In order to settle this question it was 

 necessary to realise that light is not the only condition 

 that influences growth ; it is affected also by the amount 

 of moisture, and especially by temperature. 



By growing plants in the dark and a constant amount 

 of moisture, and varying only the temperature, we find 

 that they grow more quickly at a high and more slowly at 

 a low temperature. If we make the temperature rise 

 and fall alternately, we shall notice that the index of the 

 recording apparatus described above will draw upon the 

 surface of the cylinder a line similar to the line a, b, c, d, e,f 

 (fig. 56) ,where every more steeply inclined part of the line 

 will correspond to a warmer interval and every more 

 sloping part to a colder one. This means that heat 

 acts in an opposite way to light : while light retards 



