SEASONAL PERIODICITY OF VEGETATIVE GROWTH 645 



daytime rates of elongation are more rapid on clouds' than on clear daj'S. 

 In many species, including maize (Loomis, 1934) transit of a cloud across 

 the sun permits a temporary acceleration in growth rate. 



Under some conditions maximum rates of elongation of plant organs may 

 occur in the daytime rather than at night. In the early spring, for example, 

 low night temperatures may inhibit or greatly retard the elongation of the 

 leaves of such species as wheat and blue grass which grow at this season, 

 while elongation occurs at more rapid rates during the warmer daylight hours. 

 Because of the retarding effect of the light factor such a daily periodicity of 

 elongation is most likely to occur when relatively warm, cloudy days are ac- 

 companied by cool nights. 



Seasonal Periodicity of Vegetative Growth. — All plants exhibit more 

 or less clearly marked seasonal variations in the rate of vegetative growth. 



The seasonal periodicity in the vegetative growth of any species is condi- 

 tioned partly by environmental factors and partly by internal conditions. 

 Among the former temperature and water supply are especially important. 

 Some of the internal conditions which are known to play a significant part in 

 such phenomena are dormancy, internal water relations, and correlative effects 

 among organs. In temperate regions the periodic vernal resumption of growth 

 by woody perennials is one of the most spectacular biological accompaniments 

 of the march of the seasons. This topic will be discussed almost entirely in 

 terms of such woody plants, 



A discussion of the periodicity of growth in woody plants may logically 

 begin with bud formation, which starts as an integral part of the resumption 

 of growth in the spring. In most species development of buds is completed 

 or at least well advanced by midsummer, although to the casual observer 

 they do not ordinarily become conspicuous until defoliation of the twigs 

 occurs in the autumn. Cell divisions in the apical meristems enclosed within 

 the bud scales during the summer months result in the production of an 

 embryonic shoot. In many, but by no means all, species no more leaves are 

 borne on an annual shoot than have developed in rudimentary form in the 

 bud during the preceding summer. Each bud on a woody plant therefore 

 represents essentially an immature annual shoot. 



Buds do not normally develop during the season they are produced but, 

 with certain exceptions to be noted shortly, remain for some time in a dor- 

 mant state. As shown in the preceding chapter the length of time that buds 

 on woody plants remain dormant varies greatly according to species. Some 

 lose their dormancy early in the autumn; others retain it until late in the 

 winter. The buds of temperate zone woody plants seldom open as soon as 

 they lose their dormancy but remain in a quiescent state until the favorable 



