V Movements of Plants 79 



In regard to such experiments, however, critics have justly 

 pointed out that we must be exceedingly careful, with plants 

 as well as with animals, in drawing conclusions as to normal 

 life from facts observed when injuries are inflicted, however 

 apparently slight these injuries may be ; and this experiment 

 must therefore be given up. 



Besides being affected by gravity and by light, roots are 

 very sensitive to moisture. They always grow towards the 

 greater moisture, sometimes even against gravity. Thus if 

 seeds be allowed to germinate in a sieve filled with damp 

 sawdust, the roots first grow downwards as usual, but after 

 they have descended through the sieve into the dry air, the 

 direction of their growth changes, and they grow up again 

 into the moisture. 



Light - seeking and Light - avoiding Movements. — 

 Every one must have noticed that plants which have grown 

 in the recess of a narrow window are often very markedly 

 curved towards the light. The same light-seeking can be 

 readily shown by experiment, for if seedlings are grown in 

 a box which is illumined by a single aperture they all turn 

 their young stems towards the path of the light. Some 

 sedentary animals, such as the Serpula worms, which make 

 for themselves twisted tubes of lime, have the same habit 

 of bending to the light. 



Some of Darwin's observations show the extreme sensitive- 

 ness of certain seedlings to light. " The cotyledons of 

 Phalaris became curved towards a distant lamp, which 

 emitted so little light that a pencil held vertically close to 

 the plants did not cast any shadow which the eye could 

 perceive on a white card. These cotyledons, therefore, 

 were affected by a difference in the amount of light on 

 their two sides, which the eye could not distinguish." Dar- 

 win also noticed that if seedlings kept in a dark place were 

 laterally illuminated by a small wax taper for only two or 



