Mar. 1, 1920 Effect of Length of Day on Plant Growth 579 



RELATIONSHIPS BETWEEN ANNUALS, BIENNIALS, AND PERENNIALS 



It is well known that there are no hard and fast lines of distinction 

 separating annuals, biennials, and perennials; for plants may change 

 from one of these types to another under influences of environment, 

 although in the past the particular factors of the environment involved 

 have not for the most part been understood. The experiments recorded 

 in this paper make it clear that in any particular region the relative 

 lengths of the days and nights running through the year constitute one 

 of the controlling factors in determining the behavior of plants in this 

 particular. The soybean is commonly regarded as a typical annual in 

 that its entire life cycle is completed in a single season, and coincident 

 with or soon following the maturation of the seed the plant as a whole 

 perishes. As recorded on page 567, however, a suitable change in the 

 length of the daily exposure to light revived the vegetative life of the 

 matured plants. After the first crop of seed had ripened and the foliage 

 had yellowed just as usual immediately before the plant died, new shoots 

 developed on the old stems, vegetative activity was resumed, and, finally, 

 with the approach of the shorter days of autumn, the plants blossomed 

 and fruited a second time. Thus, under controlled conditions the plant 

 simulated the behavior of a flowering perennial except that the two cycles 

 of alternate vegetative and reproductive activity have been crowded 

 into a single season. Ragweed behaved in essentially the same manner. 

 To make the analogy more convincing, attention is directed to the fact 

 that aster, a flowering perennial, under the same treatment gave exactly 

 the same results as the soybeans and ragweed. Thus, the aster readily 

 completed two complete annual cycles within a period of about four 

 months, except that, in the absence of low temperature, the original 

 growth above ground, of course, was not killed. Moreover, in the second 

 period of vegetative activity new shoots were sent up from the roots in 

 addition to the new axillary shoots appearing on the original stems. 

 The first flowering and fruiting of the soybeans, ragweed, and aster were 

 forced by artificially shortening the length of the day. When the plants 

 were restored to the full exposure of the normal summer day, vegetative 

 activity was resumed, and, finally, the natural shortening of the days in 

 August and September resulted in the second flowering and fruiting 

 periods. The factor of the environment which makes the cycle of al- 

 ternating vegetative and reproductive activities an annual event would 

 thus seem to be the annual periodicity in the length of day. If tem- 

 perature diff"erences are assumed to be the primary factor, annual perio- 

 dicity in tropical regions (not including the immediate vicinity of the 

 equator) is not readily explainable. 



As has already been pointed out, the Mammoth or giant type of 

 tobacco behaves as a typical flowering annual, like the ordinary tobaccos, 

 when grown under the influence of days not exceeding 12 hours in length. 

 During the winter months the plant blossoms readily and, in fact, becomes 

 practically an ever-blooming type. It is an interesting fact, however. 



