596 Journal of Agricultural Research voi, xvni, no. k 



This plant normally blooms from late July to middle or late September, 

 indicating that blossoming becomes more or less inhibited as the autumnal 

 equinox is passed in late September and the length of the day falls below 

 1 2 hours. In the greenhouse at Washington the short days of the winter, 

 ranging around 9 to lo hours in length, have completely inhibited the 

 flowering phase of development of this plant. The shorter 7-hour daily 

 exposures to light under controlled conditions have produced identical 

 results. Thus it appears that the normal flowering period of Mikania 

 scandens even in the warmer portions of its range should not occur much 

 later in the season than the period when the days are not less than 12 

 hours in length. This seems to be the case in Florida, where the blooming 

 season of Mikania is confined to August and September, as it is in much 

 more northern portions of its range. Plants of this type, attaining their 

 best development under daylight lengths of approximately 12 hours, 

 should also find a more or less congenial environment under truly tropical 

 conditions where the days are never much less than 1 2 hours in length. It 

 is probable that in the Tropics, however, many plants of this type would 

 not only become perennial in their aerial portions, but would also have 

 a more or less continuous flowering period. 



Since it has been shown that the stature of some plants increases in 

 proportion to the length of the day to which the plants are exposed under 

 experimental conditions, this factor should be expected to have some 

 influence upon the stature of such plants in their normal habitat. In 

 general, exceptional stature would be attained in those regions in which 

 a long day period allowed the plants to attain their maximum vegetative 

 expression before the shorter days intervened to initiate the reproductive 

 period. This condition should hold true not only for different latitudes 

 where a plant has an extensive northward and southward range but for 

 different sowings in the same locality at successively later dates during 

 the season. It is a matter of common observation that the rankest 

 growing individuals among such weeds as the ragweed, pigweed (Amaran- 

 thus), lamb's quarters (Chenopodium), cocklebur (Xanthium), beggar- 

 ticks (Bidens), other conditions being equal, are those which germinated 

 earliest in the season, and consequently were afforded the longest favor- 

 able period of vegetative activity preceding the final flowering period. 

 It is also a matter of common observation that all these weeds, when 

 germinating very late in the summer and coming at once un-der the 

 influence of the stimulus of the shortening days, blossom when very 

 small, often at a height of only a few inches. 



Many species of plants have an extensive northward and southward 

 distribution. In these instances it may be that such species are capable 

 of reacting successfully to a wide range of different lengths of day, or it 

 is possible that the apparent adjustment to such a wide range of conditions 

 may depend upon slightly different physiological requirements of different 

 types which have been developed as a result of natural selections. It yet 



