252 LECTURES TO SCIENCE TEACHERS. 



temperature at which the seeds of these plants have been 

 observed to germinate is 38 C. the temperature at which 

 germination is most active being from 20 to 25 C. 

 Experiments of this kind performed upon the seeds of a 

 variety of plants show that the range of temperature 

 through which germination may take place, extends from 

 to 50 C. 



The next piece of apparatus (No. 3936) is quite classical 

 in the history of Vegetable Physiology. It is constructed 

 and exhibited, like the preceding, by Prof. Dr. F. Cohn, of 

 the University of Breslau, and is a modification of the 

 original apparatus invented by Knight in the year 180G. 

 It consists of a large tin box in which is an axle bearing 

 a water-wheel at one end and a disc of cork at the other. 

 A stream of water is introduced through the roof of the 

 box which sets the water-wheel in motion, causing, at the 

 same time, rotation of the disc of cork, into which are 

 fixed several pins, each of which transfixes the cotyledons 

 of a pea. This apparatus has been in action for the last 

 few days, and as you see, tho peas have germinated. I 

 would ask you particularly to notice that in each case the 

 young stem has grown inwards towards the centre of the 

 axle, whereas the young root has grown outwards in the 

 opposite direction. 



Knight was led to invent this apparatus from a con- 

 sideration of the fact that the young root of a germinating 

 seed always tends to grow towards the centre of the earth, 

 whatever the position of the seed may be, whereas the 

 young stem always grows in the opposite direction. 

 Duhamel had already shown that, if the position of a seed 

 be reversed several times whilst it is germinating, the 

 young root and stem soon resume the original direction of 

 their growth after each change of position. These pheno- 

 mena were ascribed at that time to the action of gravity 

 upon the growing tissues, but these views were purely 

 hypothetical, resting upon no experimental basis : it, 

 remained for Knight to provide the necessary evidence. 



By means of this apparatus, the stem and root of the 

 young plant are exposed to precisely similar conditions of 

 light, moisture, air, and temperature, the action of gravity 

 is eliminated, and the action of a considerable centrifugal 

 force is introduced. Under these circumstances we must 



