236 



SCIENCE 



[N. S. Vol. XXXVIII. No. 972 



are here included 365 species. Of these 15 

 species are gymnosperms ; 10, palms ; 23, oaks ; 

 ■with 48 species of Crataegus. 



These three books are published by the 

 author. 



SHORT NOTES 



A NEW edition of the " Guide to the Spring 

 Flowers of Minnesota " (by Clements, Eosen- 

 dahl and Butters) has just appeared, so 

 broadened and extended as to include the 

 plants that ordinarily blossom by the middle 

 of June. Small but helpful figures of about 

 160 genera are now given in the text. The 

 plan of these " Guides," of which half a dozen 

 have been published, is to be highly com- 

 mended. 



Announcement is made of the early appear- 

 ance of a book on " Rocky Mountain Flowers," 

 by F. E. and E. S. Clements. It is to be " an 

 illustrated guide for plant-lovers and plant 

 users " and is to contain twenty-five colored 

 plates, and about as many uncolored. An 

 examination of some of the colored plates 

 indicates that they will be highly artistic as 

 well as botanically accurate. The volume is 

 bound to be one that will appeal strongly to 

 those who " summer " in the Eocky Mountains. 

 Chaeles E. Bessey 



The University op Nebraska 



SPECIAL ABTICLES 



THE APPLICABILITY OF THE PHOTOCHEMICAL 

 ENERGY-LAW TO LIGHT REACTIONS IN ANIMALS 



It has been pointed out by Loeb that tro- 

 pistic light reactions in animals should follow 

 the law of Bunsen and Eoscoe. This law 

 states that in a light reaction the eiiect is 

 proportional to the simple product of intensity 

 and time. It was first proved to be true for 

 the formation of hydrochloric acid from chlo- 

 rine and hydrogen and for the blackening of 

 silver chloride under the influence of light. 

 Later it was found to apply to the phototropic 

 curvature (Froschel, Blaauw) of plants, as 

 well as to the human eye, though within 

 rather narrow limits (Bloch, Charpentier). 

 For light reactions in animals it has fre- 

 quently been stated that they do not follow 

 this simple law. A large number of forms 



seem to react to changes of intensity only, the 

 effect in this case being proportional to the 

 amount of change per unit of time. This is 

 particularly true of the stimulating and in- 

 hibitory reflexes of the locomotor apparatus, 

 as shown by a large number of investigators. 

 It occurred to me that it might be possible 

 to get proof for the applicability of the 

 energy-law by using a reaction which did not 

 involve the locomotor organs. The eye move- 

 ments of Daphnia seemed to afford a suitable 

 object for the study of this question. These 

 movements were first observed by Eadl and 

 his observations were confirmed and extended 

 by myself some years later. The spherical 

 eyeball containing a number of radially ar- 

 ranged ocelli is capable of rotation and held 

 in position by several thin muscles inserted at 

 its periphery. The eye shows a definite nor- 

 mal position with regard to light, a certain 

 axis of the sphere having to be placed in such 

 a direction that the ocelli on all sides of this 

 axis get an equal amount of illumination. 

 The muscles keep the eye in this position and 

 one can cause rotating movements of the eye- 

 ball, by shifting the position either of the 

 source of light or of the animal. The eye will 

 always maintain its fixed position to the 

 source of light, no matter whether the body 

 of the animal follows the eye or not. An 

 unequal state of tension of the eye muscles 

 seems to cause locomotor movements, which 

 tend to restore the normal relative position of 

 eye and body. By fixing the animal on a 

 slide it can be prevented from moving and 

 the eye movements may be observed at leisure. 

 Instead of shifting the position of the light 

 the eye can be placed in a position of equilib- 

 rium between two sources of light and eye 

 movements can be caused by increasing or 

 decreasing the intensity of either of them. 

 This shows these movements to be a function 

 of the intensity of illumination. 



In order to test the energy law, it is neces- 

 sary to combine different light intensities with 

 different times of exposure. If the product 

 of time and intensity, i. e., the amount of 

 radiant energy brought to bear on the eye, is 

 the same, the eye will always give the same 



