Mar. 1. 1920 Effect of Length of Day on Plant Growth 603 



stimulus of certain day lengths. It has occurred to the writers that 

 possibly the migration of birds furnishes an interesting illustration of this 

 response. Direct response to a stimulus of this character would seem 

 to be more nearly in line with modern teachings of biology than are 

 theories which make it necessary to assume the operation of instinct or 

 volition in some form as explaining the phenomena in question. 



CONCLUSION 



The results of the experiments which have been presented in this 

 paper seem to make it plain that of the various factors of the environment 

 which affect plant life the length of the day is unique in its action on 

 sexual reproduction. Except under such extreme ranges as would be 

 totally destructive or at least highly injurious to the general well-being of 

 the plant, the result of differences in temperature, water supply, and 

 light intensity, so far as concerns sexual reproduction, appears to be, at 

 most, merely an accelerating or a retarding effect, as the case may be, 

 while the seasonal length of day may induce definite expression, initiating 

 the reproductive processes or inhibiting them, depending on whether 

 this length of day happens to be favorable or unfavorable to the particular 

 species. In broad terms, this action of the length of day may be tenta- 

 tively formulated in the following principle: Sexual reproduction can 

 be attained by the plant only when it is exposed to a specifically favorable 

 length of day (the requirements in this particular varying widely with the 

 species and variety), and exposure to a length of day unfavorable to 

 reproduction but favorable to growth tends to produce gigantism or in- 

 definite continuation of vegetative development, while exposure to a length 

 of day favorable alike to sexual reproduction and to vegetative develop- 

 ment extends the period of sexual reproduction and tends to induce the 

 "ever-bearing" type of fruiting. 



The term photoperiod is suggested to designate the favorable length of 

 day for each organism, and photo periodism is suggested to designate the 

 response of organism to the relative length of day and night. 



SUMMARY 



(i) The relative length of the day is a factor of the first importance 

 in the growth and development of plants, particularly with respect to 

 sexual reproduction. 



(2) In a number of species studied it has been found that normally 

 the plant can attain the flowering and fruiting stages only when the length 

 of day falls within certain limits, and, consequently, these stages of 

 development ordinarily are reached only during certain seasons of the 

 year. In this particular, some species and varieties respond to relatively 

 long days, while others respond to short days, and still others are capable 

 of responding to all lengths of the day which prevail in tlie latitude of 

 Washington where the tests were made. 



