646 GROWTH PERIODICITY 



environmental conditions of spring. Low temperature is probably the prin- 

 cipal factor preventing development of quiescent buds in the late winter and 

 early spring, although a deficient water supply may also be involved, at least 

 in some species. Photoperiodic effects may also play a part in the vernal re- 

 sumption of vegetative growth by plants. 



The development of new shoots from the buds on woody stems in the 

 spring is by no means always a continuous process. In species such as cherries 

 and willows which "leaf out" relatively early, the growth process is often 

 intermittent. During this season periods of warm weather often alternate 

 with colder spells. Hence elongation of the developing shoots may take 

 place in a series of short spurts, each terminated upon the advent of un- 

 favorably cool weather. Apical growth is much more likely to proceed unin- 

 terruptedly in species such as beech and the hickories in which it is initiated 

 later in the spring. Under favorable conditions practically all stem elonga- 

 tion and development of the new crop of leaves may occur in such species dur- 

 ing a growth period lasting only two or three weeks. Termination of the 

 spring burst of growth in such species is evidently due to internal causes, since 

 environmental conditions usually remain favorable for growth during much 

 or all of the summer. 



The stems of some ligneous species (sumac, dogwood, ailanthus, etc.) 

 do not grow in the definite manner described above, but continue to elongate, 

 producing leaf after leaf, for most or all of the summer, quiescent intermis- 

 sions occurring only when environmental conditions are unfavorable. In some 

 such species growth is not terminated until the advent of frost. 



Under some conditions the buds on woody stems open the same season 

 they are formed. This is a commoner occurrence in some species than in 

 others, and is more likely to happen on young trees or shrubs than on old ones. 

 Defoliation of a tree relatively early in the season usually results in a resump- 

 tion of growth from buds developed during the current season. During wet 

 summers development of the currently produced buds into shoots occurs fre- 

 quently in many species of woody plants. Such shoots are often called 

 "Lammas-shoots." ^ Among the oaks, especially when young, the production 

 of two or even more successive shoots during a growing season is a common 

 occurrence. When the terminal bud on the stem of an oak resumes growth 

 during the season it is produced, lateral buds on that same segment also 

 usually resume growth and produce side branches. In the willow oak 



1 So-called because they are supposed to develop about August first, which 

 according to the church calendar is "Lammas-day." Actually Lammas shoots 

 usually develop earlier in the summer than this, at least under North American 

 conditions. 



