584 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



showed the typical everbearing behavior. These latter plants began 

 flowering somewhat later; they continued to flower and fruit some- 

 what sparingly throughout the winter, continuing to grow all the 

 while, the final height being about 10 feet. This is in line with the 

 well-known fact that buckwheat succeeds best in northern latitudes, 

 where the summer days are long. It has already been pointed out 

 that by the use of electric lights to lengthen the short winter days 

 spinach was promptly forced into flowering, and it may be added 

 here that under the influence of the lengthened day, which was main- 

 tained at approximately constant duration, the spinach continued to 

 flower for several weeks, thus approaching the everbearing condition. 

 Again, by maintaining a constant day-length of about eight hours 

 violets have been caused to produce purple, petaliferous blossoms 

 continuously for several months. The fact is that under a suitable 

 and approximately constant day-length the everblooming habit is 

 the rule rather than the exception, but in nature everblooming is con- 

 fined to comparatively few species and varieties growing outside 

 the Tropics, because the proper length of day does not continue for 

 the necessary length of time. Some species are able to grow and to 

 flower simultaneously through a considerable range in day-length, 

 but for most species the seasonal change in length of day in temperate 

 regions is too rapid to allow this combined type of development to 

 come into expression. 



REJUVENESCENCE IN PLANTS. 



To ascertain the cause of senility and resultant death, which always 

 follows sooner or later, has long been one of the great problems of the 

 biologist, and as yet no satisfactory solution of this problem has been 

 offered. In plants as in animals the average length of life of differ- 

 ent species differs enormously. Considering only the higher plants, 

 there are many which spring up from the seed, attain their full 

 growth, flower, ripen their seed, and perish within a period of a few 

 weeks. Again, there are numerous species which live for hundreds 

 of years and a few which continue to live even for thousands of 

 years. One large and important group are known as annuals for the 

 reason that they usually mature seed and perish within a year's 

 time from germination, though the life cycle is not necessarily coin- 

 cident with the calendar year. One of the most striking features 

 of the autumn landscape which so clearly marks the approaching 

 close of the principal growing season for plant life is furnished by 

 the slowing down of growth, the development of seed, shedding or 

 withering of leaves, and other indications of approaching death in 

 the summer annuals and of transition into dormancy or the winter 

 rest period on the part of the corresponding types of perennials. 

 It is commonly considered that the death of these annuals in the 



