Mar. 1, 1920 Effect of Length of Day on Plant Growth 581 



action of relative day length is the behavior of certain northern varieties 

 of pepper (Capsicum) when planted in Porto Rico in the spring months 

 (13). Under these conditions the peppers imported from the higher 

 latitudes of the United States were able to form only a very few fruits 

 before they began to yellow and shed their foliage, after which the 

 plants soon perished. Also, it is stated that the radish when grown in 

 Porto Rico during the winter months behaves as it does when grown 

 nearer the equator. The above-mentioned experimental results and 

 observations seem to justify the conclusion that the relative length of 

 the day through the year is a factor of the first importance in determin- 

 ing whether many plants behave as annuals, biennials, or perennials, 

 and whether reproduction in such plants is vegetative or sexual or both 

 in any particular region. 



The forcing of two flowering periods in a single season under con- 

 trolled conditions naturally directs attention to another phase of 

 periodicity in plant activity — namely, the appearance of the blossoming 

 period in both spring and fall, or only in one of these seasons in regions 

 outside the Tropics. This question is of special interest with respect 

 to perennials. It is apparent that plants blooming only in the spring 

 or fall or in both seasons are to be regarded as requiring relatively short 

 days for attaining this stage. In annuals, ordinarily a period of vege- 

 tative development must necessarily precede flowering, so that the latter 

 stage is likely to be deferred till autumn ; but when propagation is by means 

 of bulbs or other reproductive storage organs, blossoming may well occur 

 in the spring. In hardy shrubs and trees a typical condition is that 

 in which the formation of flowers or flower buds is inaugurated in the 

 autumn under the influence of the shortening days, while the flowering 

 process is interrupted before completion through the inter\^ention of 

 cold weather. The result is that actual blossoming usually takes 

 place in the spring; but if the fall or early winter temperatures are abnor- 

 mally high, the flowering process may be completed before cold weather 

 intervenes. This phenomenon is occasionally observed in the apple. 

 In the spring, temperature would be the chief factor in determining 

 the date of blossoming for this class of plants. It is suggested that the 

 seasonal distribution of flower-bud formation in the lemon which is 

 considered in a recent interesting article by Reed (22) may be due to 

 these light and temperature relations. The process is most active 

 during the late fall and again in very early spring, with a winter period 

 of low activity. Throughout the summer period of long days, also, 

 activity is at a minimum. 



LENGTH OF DAY CONTRASTED WITH LIGHT INTENSITY 



As early as 1735 Reaumer (21) undertook to make accurate compari- 

 sons of the total quantities of heat required to bring plants to given 

 stages of maturity. At intervals since that time this idea has been 



