186 PLANT GROWTH 



has tried, or has heard of our friends trying, the chemical 

 vitamin Bi, as a growth stimulant. The importance of a soil 

 with a texture loose enough to supply oxygen to the roots 

 must be included here and a reference to other growth-regu- 

 lating conditions described in earlier chapters. 



Plants grow in cycles or at certain periods, alternating 

 with periods of rest or dormancy, as is commonly observed 

 in the spring after the winter dormancy. This is one of the 

 hereditary characteristics. It is true for the shoot and for 

 the root, although the first period of root growth begins 

 before shoots elongate and the leaves grow in the spring and 

 a second period occurs in many plants in the fall. The 

 growth in stem thickness may be more variable depending 

 on the growth conditions but normally the stem thickens 

 more rapidly during the spring growing period. Flowering 

 cycles in some plants depend on length of day rather than 

 time of year or the age of the plant. Any experimental work 

 should be planned to coincide with the normal growth cycle. 



It is possible in many cases to modify the growth cycle, 

 but that at once becomes an experiment of major impor- 

 tance. The reader may refer to some of the work of F. E. 

 Denny and his co-workers, reported in the contributions of 

 the Boyce Thompson Institute, for the details of the treat- 

 ments. Gardeners make use of natural growth cycles in fol- 

 lowing the periods of planting, pruning, and harvesting the 

 desired crop. 



One of the most striking and interesting illustrations of 

 changing the normal growth cycle was announced by the 

 Russians a few years ago concerning wheat. Normally 

 sown in the fall, winter wheat grows a small plant before 

 winter puts it in a dormant condition. The next spring's 

 long days induce heading and ripening. "Vernalization" is 

 the term applied to forcing plants to go through a part of 

 their life cycle in an unusual controlled method. The treat- 

 ment of wheat consists in giving a mass or pile of it about 

 half the maximum amount of water it can absorb, and hold- 



