LUNAR PERIODICITY IN LIVING ORGANISMS 281 



Order Cucurbitaceae grow most rapidly on moonlit nights. 

 An investigation of the rate of growth of these fruits, extended 

 over three months, has shown me, however, that there is no 

 basis at all for this behef. As the lunar influence is supposed 

 to show itself especially in these cucurbitaceous fruits, it may 

 safely be assumed to be absent in all fruits. 



Nevertheless, since the popular belief in the existence of a 

 lunar cycle in sea-urchins has turned out to be so unexpectedly 

 true, one is bound to ask whether any other of the superstitions 

 about plants too are based on fact. It is conceivable, for instance, 

 that moonlight may have a photosynthetic effect. Kofoid 

 {Bull. Illinois State Lah. Nat. Hist., 8, 1908) found a lunar perio- 

 dicity in the frequencies of plankton organisms in the Illinois 

 River. Allen {Univ. Cal. Pubs. Zool., 22, 1920) discovered the 

 same thing in the San Joaquin River, California. The maximum 

 frequency of Algse occurred at full moon, that of Crustacea a little 

 later. While the crustacean presumably follows the algal maxi- 

 mum because the animals feed on the plants, Kofoid attributes the 

 algal maximum occurring about full moon to a photosynthetic 

 effect of moonlight. He supports this hypothesis by reference 

 to experiments of Knaute (5io/. Centralbl., 18, 1898), who states 

 that he found the oxygen content of water containing Euglena 

 to be higher in moonlight than in darkness, which means that 

 the moonlight causes photosynthesis. Knaute says that the 

 photosynthetic effect of moonlight is to that of sunhght as 2 : 9. 

 This is a surprisingly high ratio, since the light of the sun is 

 about 600,000 times that of the full moon. In view of the great 

 theoretical importance of lunar photosynthesis, if it really exists, 

 I am at present repeating Knaute's work. Preliminary experi- 

 ments, using, not an Alga, but the aquatic flowering plant 

 Elodea, have shown that here the lower limit of illumination 

 necessary for photosynthesis is of a totally different order of 

 intensity from even full moonlight. The method used was 

 based on the change in hydrogen ion concentration in the water 

 in which the plant was placed, due to the change in its carbon 

 dioxide content. 



There are, however, two demonstrated effects of moonlight 

 upon plants. Firstly, Musset (C. R. Acad. Sci., 1883 and 1890) 

 showed that a number of flowering plants are positively photo- 

 tropic to moonlight. The stems bend towards the moon and 

 during the course of the night move round to follow his path. 

 Secondly, Loftfield {Carnegie Inst. Publ., 314, 1921) states that 

 at night stomata open as a result of illumination by moonlight. 

 The consequent periodic opportunity for increased transpiration 

 and respiration might conceivably cause other rhythmic changes 

 in the plant, with a lunar period. 



In conclusion, two cases of lunar effects on the migration 



