SEASONAL PERIODICITY OF VEGETATIVE GROWTH 647 



{Quercus phellos) three or even more prolongations of the same woody axis 

 may take place during a single growing season. Almost always, howev^er, 

 when buds of the current crop on woody stems resume growth there is a short 

 dormant period between the time formation of the bud is completed and the 

 time its active growth is resumed. 



Cambial growth as well as the development of the terminal bud meristems 

 is also characterized by a marked seasonal periodicity'. Division of the cam- 

 bium cells in woody stems usually does not start until after expansion of the 

 buds has begun. In many and perhaps all tree species resumption of growth 

 by the stem cambium begins just beneath the enlarging buds and then pro- 

 gresses in a basipetal direction. Cambial activity spreads gradually down the 

 twigs, from the twigs into the branches, from the branches into the trunk 

 and ultimately into the roots (Priestley, 1930). The numerous "waves" 

 of cambial growth which are initiated in this way in the smaller branches 

 apparently become more or less integrated into a single wave as they pass 

 downward into the main axis of the plant. Hence in large trees radial growth 

 may not be resumed in the trunk until several weeks later than in the small 

 branches, and in the roots not until several weeks later than in the trunk. In 

 the roots, however, some cambial growth apparently occurs independently of 

 cambial activity in the stems and trunk. 



Suggestions that the downward propagation of a cambial stimulus through 

 the stems of woody plants in the spring may be accounted for in terms of 

 the basipetal movement of hormones have found some experimental support. 

 According to Snow (1935) when aqueous solutions of auxin a or indole-3 

 acetic acid (heteroauxin) were applied to the terminal ends of decapitated 

 sunflower seedlings, division of the cambial cells was initiated. Recent work 

 by Avery, et al. (1937) also points to this conclusion. Buds of the horse 

 chestnut and apple in the winter condition failed to give any indication of 

 the presence of active hormones when tested by the oat coleoptile technique. 

 As the terminal buds of both of these species swelled, growth hormone was 

 found to be present in increasing concentrations. The peak concentration 

 was found just prior to the most rapid elongation of the current season's 

 shoots. Hormones were produced both in terminal buds and the current 

 season's shoots. Cambial activity began just below the terminal buds and 

 progressed basipetally into older portions of the stem, following movement 

 of the hormone into those regions. It has also been shown that introduction 

 of a crystal of indole-3 acetic acid into the cambium of willow and other 

 woody species leads to rapid cambial growth in a short zone below the point 

 of insertion of the crystal (Soding, 1936). These observations make it seem 



