648 GROWTH PERIODICITY 



highly probable that vernal resumption of cambial activity occurs under the 

 influence of a hormonal stimulus. 



Secondary thickening of the stems of most woody species generally con- 

 tinues until later in the summer than elongation of the current shoots, al- 

 though usually at a diminishing rate. Cambial activity usually ceases in 

 the young twigs by midsummer, but may continue until late summer or early 

 autumn in the older stems and sometimes until winter in the roots. Cessation 

 of cambial activity in stems during the summer months may often be causally 

 related to the decrease in water content of the stem tissues which usually 

 takes place at that season. 



Less is known regarding the seasonal periodicity of the growth of roots 

 than of the aerial organs of plants. The elongation of the roots of woody 

 species apparently begins somewhat earlier in the spring than the elongation 

 or cambial activity of stems. Growth of roots both in length and in diameter 

 continues later into the autumn than the growth of stems. Elongation of 

 roots even through the winter months has been reported by several observers. 

 Harris (1926), for example, records continued growth of apple and filbert 

 roots in Oregon during the winter months. A temporary midsummer slacken- 

 ing in the rate of root elongation has been observed in some species (Stevens, 



1930- 



Cyclical Periodicity of Vegetative and Reproductive Growth. — The 



examples of seasonal periodicity which have already been described involve 

 principally variations in growth rates. Growth periodicity is expressed not 

 only in terms of seasonal variations in the quantitative aspects of growth, 

 but also in the production of certain organs at one stage in the life cycle 

 and other organs at another stage. The most prominent periodicity in the 

 qualitative aspects of plant growth is the cyclical production of vegetative 

 and reproductive organs which is exhibited by most species of plants. 



The seasonal periodicity of all annual species is very similar, and involves 

 in sequence: (i) seed germination, (2) vegetative development, (3) flower- 

 ing and fruiting, usually accompanied, at least during the later stages, by 

 slowly diminishing vegetative growth, (4) senescence, and (5) death of all 

 organs except the seeds. All such species are perennial only by their seeds. 



The seasonal periodicity of annual species is by no means immutable, 

 however, but can be altered in various ways. Removal of flowers or fruits 

 or both often leads to an acceleration or renewal of vegetative growth (Chap. 

 XXXIV). Similarly change in the length of the photoperiod at the onset 

 of senescence often causes a reiuvenation of vegetative growth (Chap. 

 XXXIII). 



The cyclical production of vegetative and reproductive organs is very 



