594 Journal of Agricultural Research vo1.xviii.no.ii 



consideration. Heretofore temperature, water, and light intensity rela- 

 tions have been considered the chief external limiting factors governing 

 the distribution or range of plants. In the light of the observations and 

 experimental results presented in this paper it seems probable that an 

 additional factor, the relative length of the days and nights during the 

 growing period, must also be recognized as among those causes under- 

 lying the northward or southward distribution of plants. ^ It is evident 

 that the equatorial regions of the earth alone enjoy equal days and 

 nights throughout the entire year. Provided the water relations are 

 favorable, the warm temperatures in these regions favor a continuous 

 growing season for plants. Passing northward from the equatorial 

 regions into higher latitudes, temperatures promoting active vegetative 

 growth and development are restricted to a summer period which, other 

 conditions being equal, becomes progressively shorter as the polar 

 regions are approached. Coincident with these changes from lower to 

 higher latitudes, the summers are characterized by lengthening periods 

 of daylight and the winters by decreasing periods of daylight. We may 

 now consider how these dififerent day and night relations operating 

 during the summer growing period will exercise more or less control upon 

 the northward or southward distribution of certain plants. 



It is evident that a plant can not persist in a given region or extend 

 its range in any direction unless it finds conditions not only favorable 

 for vegetative activity but also for some form of successful reproduction. 

 For present purposes only sexual or seed reproduction need be considered. 

 The experiments above described have indicated that for certain plants — 

 for example, ragweed and the aster — the reproductive or flowering phase 

 of development in some way depends upon a stimulus afforded by the 

 shortening of the days and the consequent lengthening of the nights as 

 the summer solstice is passed. It remains to consider more specifically 

 the bearing these facts may have when plants characterized by this type 

 of behavior are subjected to the daylight relations of different latitudes. 

 In the vicinity of Washington, D. C, the ragweeds regularly shed their 

 first pollen about the middle of August. It may be considered that the 

 earliest flowering plants bloom about this date each season because they 

 react to a length of day somewhat less than that of the longest day, which 

 is about 15 hours in this latitude. In other words, as soon as the 

 decreasing length of day falls somewhat below 15 hours, a condition 

 which obtains about July i, the period of purely vegetative activity is 

 checked, and the flowering phase of development is initiated. Should 

 the seeds of such plants now be carried as far as northern Maine into a 

 latitude of 46° to 47°, these plants would not experience a length of day 

 falling below 15 hours in length, for which it is assumed they are best 



' In this connection the tables showing the time of simrise and sunset at lo-day intervals through the 

 year for various latitudes in North America, as given in Smithsonian contributions to knowledge, 

 V. 21 (1876), p. 114-119, will be found very convenient for reference. 



