Februakt 3, 1911] 



SCIENCE 



167 



turn automatically to or from the source of 

 light, depending on whether they are posi- 

 tively or negatively heliotropic. In some 

 cases they also move automatically towards 

 or away from the light. Loeb-^ says in re- 

 gard to this : 



This automatic orientation is determined by 

 two factors, first a peculiar photo-sensitiveness 

 of the retina (or skin), and second a peculiar 

 nervous connection between the retina and the 

 muscular apparatus. In symmetrically built 

 heliotropic animals in which the symmetrical 

 muscles participate equally in locomotion, the 

 symmetrical muscles work with equal energy as 

 long as the photo-chemical processes in both eyes 

 are identical. If, however, one eye is struck by 

 stronger light than the o.her, the symmetrical 

 muscles will work unequally and in positively 

 heliotropic animals those muscles will work with 

 greater energy which bring the plane of sym- 

 metry back into the direction of the rays of light 

 and the head towards the source. As soon as 

 both eyes are struck by the rays of light at the 

 same angle, there is no more reason for the 

 animal to deviate from this direction and it wiu 

 move in a straight line. All this holds good on 

 the supposition that the animals are exposed to 

 only one source of light and are very sensitive to 

 light. 



Loeb^° has also shown that the helio- 

 tropism may sometimes be modified or even 

 reversed by adding certain chemicals to the 

 water. He has also shown^" that some ani- 

 mals seek automatically the places where 

 the intensity of light is a minimum; but 

 that this is not negative heliotropism be- 

 cause the animals do not necessarly move 

 along the path of the ray. In all these 

 cases we have an application of our law, 

 because the animals arrange themselves so 

 as to minimize the state of stress. 



It had always seemed to me a most 

 mysterious thing that animals should be 

 heliotropic, until it finally dawned on me 



^ " Darwin and Modern Science," 264, 1909. 



^ ■■ The Dynamics of Living Matter," 131, 1906; 

 " Darwin and Modern Science," 265, 1909. 



" Loeb, " The Dynamics of Living Matter," 136, 

 1906. 



that we are all of us heliotropic to a certain 

 extent. Place a man out in an intense 

 light and, if he can not shade his eyes, it 

 will take a pretty strong special stimulus 

 to keep him from turning his back to the 

 light. If his eyes happen to be weak, he 

 will respond more promptly and more au- 

 tomatically to the light. He is negatively 

 heliotropic to strong light. I am inclined 

 to think that man is positively heliotropic 

 to a faint light, because he would certainly 

 tend to turn towards the point that he 

 could see. We can also find an analogy to 

 the case of the animals which congregated 

 at the places where the intensity of light is 

 a minimum. Place a man out on the 

 desert in blazing sunlight and it will take 

 a strong counter-stimulus to keep him from 

 moving into any shade that he can find. 

 Here the irritation is due to heat and not 

 to light; but the man is not negatively 

 thermotropic because he will move across 

 the temperature gradient in order to reach 

 the shade. 



Of course the reflex action is not rela- 

 tively so powerful with man as with the 

 lower animals. In fact, Loeb^^ says that 

 "it rarely happens that animals endowed 

 with the mechanisms of associated memory 

 react in such a machine-like manner to the 

 elementary forces of nature as the helio- 

 tropic animals which we have discussed." 



Vernon''^ points out that 



If plants be allowed to grow in absolute dark- 

 ness, they, as a rule, become very much elongated 

 in form whilst their leaves are small and ill 

 shaped. . . . Sachs found that potato tubers 

 grown in darkness for fifty-three days produced 

 sprouts from 150 to 200 mm. high, whilst similar 

 ones grown in daylight were only 10 to 13 mm. 

 high. Again he found that the hypocotyl of the 

 buckwheat (Fagopyrum) reached a height of 35 

 to 40 cm. in the dark, whilst it grew only to 2 

 or 3 cm. when freely exposed to light. K. Goebel 



" " The Dynamics of Living Matter," 135, 1906. 

 ^ " Variation in Animals and Plants," 245, 

 1903. 



