PIIOTOPERIODISM 709 



unsatisfactory results in Australia, Forster and others (21) grew certain 

 varieties from each country in different lengths of day at Wisley in 

 England, Aberystwyth in Wales, and near Melbourne in Australia. The 

 Australian varieties were found to be capable of exserting spikes under 

 shorter periods of light than the British spring varieties which were later 

 under all light periods. These results may partly explain the failure 

 of British varieties in Australia where the summer-day length is much 

 shorter. In experiments with woody species, conducted at Leningrad, 

 to which reference already has been made (page 692), Mochkov found 

 that in the natural full day Salix lanata L., a representative of high 

 latitudes, ceased growth long before winter set in, while Rohinia Pseudo- 

 Acacia L., from lower latitudes, did not stop its growth till the branches 

 were killed by frost. It is concluded that frost resistance, one of the chief 

 factors determining the northward range of woody plants, depends to a 

 considerable extent on response of these plants to length of day. To 

 successfully withstand the winter they must terminate their vegetative 

 activity in conditions of the normal full day before the advent of cold. 



Since the range in the annual cycle of day length decreases with 

 decrease in latitude, it might be expected that photoperiodism would 

 become of less importance as an ecological factor as the tropics are 

 approached. However, results of tests reported by McClelland (46, 47) 

 in Puerto Rico, with an annual range in length of day only from 11 to 

 13.2 hr., would indicate that length of day is a factor of considerable 

 importance even in tropical regions. It appears that some tropical 

 species are sensitive to only slight changes in day length. Allard (4) 

 recently has pointed out that, since the greatest variation in time of 

 flowering among individuals of a population is shown near the critical 

 day length for flowering and with day lengths well removed from the 

 critical this variation largely ceases, attempts to secure strains or races 

 of plants adapted to other latitudes logically should be based upon selec- 

 tion made near the critical period of light for flowering and fruiting. 

 Laurie and Poesch (40) have done considerable experimentation in 

 utilization of the length-of-day effect in producing flowering plants out 

 of their normal season. 



REFERENCES 



1. Adams, J. The effect on certain plants of altering the daily period of light. 

 Ann. Bot. 37: 75-94. 1923. 



2. Adams, J. Duration of hght and growth. Ann. Bot. 38: 509-523. 1924. 



3. Allard, H. A. Gigantism in Nicotiana tabacum and its alternative inheritance. 

 Amer. Nat. 53 : 218-233. 1919. 



4. Allard, H. A. Length of day in relation to the natural and artificial distribution 

 of plants. Ecology 13 : 221-234. 1932. 



5. Arthur, J. M., J. D. Guthrie, and J. M. Newell. Some effects of artificial 

 chmates on the growth and chemical composition of plants. Amer. Jour. Bot. 

 17:416-482. 1930. 



