6o2 > Journal of Agricultural Research voi. xviu, no. n 



IS THE RESPONSE TO DIFFERENCES IN THE LENGTH OF DAY A 

 PRINCIPLE OF GENERAL APPLICABILITY IN BIOLOGY? 



Experience has abundantly demonstrated the fact that the biologist 

 who attempts to draw sweeping generalizations regarding responses of 

 plants or animals as a whole to conditions of the environment is in serious 

 danger of going astray, even though his observations be based on the 

 behavior of relatively large numbers of species. With this fact clearly 

 in mind, the following suggestions are put forward tentatively but as 

 possibly being of sufficient interest to justify careful consideration on 

 the part of biologists especially concerned in the fields touched upon. 

 It has been clearly brought out in this paper that for a number of plant 

 species the appropriate length of day acts, not merely as an accelerative, 

 but rather as an initiative influence in bringing into expression the plant's 

 potential capacity for sexual reproduction. Perhaps, as an equally sat- 

 isfactory way of expressing the fact, it may be said that the length of 

 the day exercises a truly determinative influence on plant growth as be- 

 tween the purely vegetative and the (sexually) reproductive forms of 

 development. The response to length of day may be expected to hold 

 for other species, although it would be premature at present to assert 

 that all higher plants will be found to respond to this factor. 



One is naturally inclined to inquire whether, also, the length of day is a 

 controlling factor in sexual reproduction among the lower forms of plant 

 life. The observed behavior of some of these lower forms certainly sug- 

 gests that they come under the influence of the seasonal range in length 

 of day. A single instance will suffice to illustrate the parallelism exist- 

 ing between the vegetative and the reproductive periods of activity, on 

 the one hand, and the periodical change in the length of the day, on the 

 other. Reference is made to the work of Lewis (14) , in which it is shown 

 that in certain species of red Algae there is a definite seasonal periodicity 

 in the appearance of sexual and asexual forms. In brief, the July growth 

 of these species consists primarily of tetrasporic or asexual individuals, 

 while through August the growth is characterized by a predominance 

 of sexual plants produced from the tetraspores of the July crop of plants. 

 The carpospores of autumn become sporelings which persist through the 

 winter and give rise to the tetrasporic plants of the early summer period. 

 Should it be true that lower plants respond to differences in length of 

 day as do some of the higher species it may be expected that various 

 relationships between annual and perennial forms, differences in sen- 

 sibility to relatively long and short days, and other facts which have 

 been shown to apply to these higher species would likewise hold true 

 for lower organisms. It is possible, even, that the seasonal activities of 

 some of the parasitic microorganisms are the result of response to changes 

 in day length. 



As to animal life nothing definite can be said, but it may be found 

 eventually that the animal organism is capable of responding to the 



