c8o Journal of Agricultural Research voi. xviii, no. h 



that as the seed capsules mature the seed-bearing stem dies back only to 

 the first node which may have sent up a new branch. This holds true 

 even though the new branch be but a few inches below the seed head. 

 The portion of the stem below the new branch and the root system 

 henceforth function as parts of a new plant. In winter the new branch 

 blossoms and fruits promptly, perishes, and is succeeded by new branches. 

 As spring advances the new branches coming out assume the giant or 

 nonflowering type of growth which continues till fall brings a return of 

 the short days, when blossoms promptly appear. It would seem that 

 the new branch acts as a rejuvenating or a protective agent against the 

 death of the older organs to which it is attached. Obviously the Mam- 

 moth tobacco resembles both the annual and the perennial types of 

 plant life. The sharpness with which the new branch controls the extent 

 of the dying-back of the mother stem is shown in Plate 76, A. 



In the latitude of Washington the radish is an annual unless planted 

 very late in the season. It has already been shown that under a short- 

 ened light exposure, on the other hand, while vegetative development 

 may continue, flowering does not occur. It would appear from this that 

 the radish might not flower in regions where the maximum length of days 

 is relatively short; and, in fact, according to Dr. Walter Van Fleet, of 

 the Bureau of Plant Industry, the radish as a rule does not blossom when 

 grown in the equatorial region. Similarly, the radish blossoms only 

 occasionally as far north as Porto Rico, where the principal growing 

 season is during the winter months {13). This behavior of the radish, 

 again, is obviously an approach toward the nonflowering type of peren- 

 nial. Similarly, Dr. Van Fleet states that a lima bean coming under his 

 observation in the Tropics had continued to grow as a perennial for a 

 number of years, having attained giant proportions, while there was 

 only occasional and sparse fruiting. Conversely, the beet ordinarily is 

 a biennial in the latitude of Washington, but when grown in Alaska 

 where the summer days are very long, it is likely to develop seed and 

 thus complete its life cycle in a single season. The intimate relationship 

 existing between the length of day and the attainment of the repro- 

 ductive stage is strikingly shown by the behavior of the radish under 

 special conditions. In the box of plants used as controls in the experi- 

 ment described on page 565 and discussed above, the great majority 

 of the individuals developed normal flowering stalks and seed pods in 

 due season (see PI. 75, B). A few individuals, however, developed con- 

 siderably later, because of delayed germination or some other reason; 

 and these delayed plants began the formation of flowering stalks. The 

 length of the day having decreased to the critical length, the growth of 

 the seed stalk was arrested after a height of a few inches was attained; 

 and instead of the normal flower head, a crown of foliage leaves devel- 

 oped, as shown in Plate 69, B, thus indicating the resumption of vegeta- 

 tive activity. What is believed to be another example of the directing 



