8o Chapters in Modern Botany CHAP. 



three minutes at intervals of about three-quarters of an 

 hour, they all became bowed to the point where the taper 

 had been held. This convinced him that the excitement 

 from the light was due not so much to its actual amount 

 as to the difference in amount between that previously 

 received. "Light seems to act on the tissues of plants 

 almost in the same manner as it does on the nervous 

 system of animals." 



Usually the movement is towards the light, especially in 

 the case of stems, but this is not invariably the case. Thus 

 the shoots of the ivy bend away from, not towards the light, 

 and so do the tendrils of the vine, of the Virginian creeper ' 

 (Ampelopsis hederacea), and some other plants. We can 

 easily understand that it is advantageous for tendrils to 

 seek shaded recesses, for in so doing they will' usually come 

 nearer the support to which they cling, although of 

 course this advantage must not blind us to the necessity of 

 a physiological explanation. 



As most roots seek the ground their reactions to light 

 are not readily tested, except in artificial conditions. But 

 by means of the klinostat the rotating apparatus which 

 we have already mentioned it is possible to grow seedlings 

 illumined in one direction only, and with the influence of 

 gravitation eliminated. Then it is seen that some roots 

 bend towards and others away from the light. Of roots 

 which in natural conditions always bend away from the 

 light, the climbing roots of the ivy are the most familiar 

 examples. Or the following simple experiment may be made, 

 following the directions of Professor Detmer's laboratory 

 manual of practical vegetable physiology a work which 

 will be of great use to the student (and which can also be 

 obtained in translation) : A glass filled with water is closed 

 with a lid of fine muslin ; on the top of this are placed 

 germinating seeds of white mustard (Sinapis alba) ; the 



