HELIOTROPIC REACTIONS — ^ANIMALS AND PLANTS 223 



light until the axis of symmetry of their photosensitive organs 

 goes through the source of light (provided only one source of 

 light is given) ; while movable plant organs, e.g., the swarmspores 

 of algae move to or from the source of light and collect on the 

 side of the light (or on the opposite side) just as do motile helio- 

 tropic animals. For the heliotropic reactions of both animals 

 and plants the validity of the law of Bunsen and Roscoe has been 

 proved'^ and the sense of heliotropic reactions in both groups 

 can be reversed by similar means. ^^ It would be artificial to state 

 that because the ones are termed animals and the others plants 

 the identical phenomena in both must be different. Such a 

 ^'iew might ha\'e been considered at the time of Linne, but today 

 we know that the mechanism of life phenomena in animals and 

 plants is essentially the same. While modern biology, especially 

 since Claude Bernard and Hoppe-Seyler, has tried to establish 

 the essential identity of life phenomena in plants and animals, 

 Hess apparently expects biologists to overlook the progress made 

 in biology and return to the Linnean viewpoint. In order to 

 maintain his artificial barrier between animals and plants he in- 

 sists that the wave length which is most efficient in the helio- 

 tropic reactions in plants is different from the one most efficient 

 in animals. But even if this were a fact, it would not justifj' 

 his assumption, since the theory of heliotropism only states that 

 organisms are automatically oriented by the light so that sym- 

 metrical elements of their photosensitive surface are struck at 

 the same angle by the light (or that symmetrical elements re- 

 ceive an equal amount of illumination during a properly chospn 

 unit of time). Whether in one case the yellowish-green, in 

 another the blue light is more efficient is secondary. 



As a matter of fact, there are heliotropic animals for which 

 the blue rays are as efficient as they are for plants; and there are 

 unicellular organisms, for which the optimum lies in different 

 parts of the spectrum. 



i^Loeb and Ewald. Centralbl. f. Physiol., 1914, 27, 1165; Ewald, Ztschr. f. 

 Psychol, u. Physiol, d. Sinnesorg., 1914, 48, Abt. 2, 285. 

 1* Loeb. Arch. f. d. ges. Physiol., 1906, 115, 564. 



