Killing of Plant Tissue by Low Temperature 279 



fruit buds only in climates like that from Central Missouri South, 

 where there is likely to be weather warm enough to start the buds 

 into growth before the effect of the rest period ends. 



It may be said also, especially concerning thin soils such as are 

 to be found in the Ozark regions of Missouri and Arkansas, if the 

 tree is permitted to bear an exceptionally heavy crop, it sets a much 

 smaller number of fruit buds. While experiments have not always 

 indicated that this is true with apples, any one who has had an op- 

 portunity to observe a considerable number of peach orchards where 

 only part of the trees have been thinned will be readily convinced 

 that it is true with peaches. This fact has its bearing on the problem 

 of killing from cold since with a very light set following a heavy crop, 

 if as many as 75 per cent of the buds are killed, there will not be 

 enough buds for a good crop, while if the trees were thinned, 25 per 

 cent of the buds would be enough for a heavy crop. By consulting 

 Table 39 it will be seen that in a large number of years as many as 

 75 per cent of the fruit buds are killed on Elberta trees. In sections 

 like South Missouri where the rest period seriously affects the amount 

 of bud killing, thinning, when necessary, would thus seem to affect 

 the size of the following crop on nearly every year. 



The greater hardiness in late winter of fruit buds on severely 

 pruned trees and on trees fertilized with nitrate, like those mentioned 

 above, represents a difference in time of ending the rest period be- 

 tween fruit buds on very vigorous trees and on trees of medium vigor. 

 Where the rest period was prolonged by thinning, the difference in 

 hardiness represents a difference in time of ending the rest period 

 between fruit buds on trees of medium vigor and on trees in a rather 

 weak condition. In case of trees making a weak growth for any 

 reason, as compared with trees making a moderately vigorous growth, 

 so far as our observations have gone, the differences while not so 

 great, are yet often apparent. In a number of years the buds or 

 blossoms have been so nearly all killed that the crop in extreme South 

 Missouri on weakly growing trees, amounted to nothing, while trees 

 of medium vigor, like an average six-year-old tree, bore good croj:)s 

 in the same years. 



In attempting to increase the hardiness of peach buds under 

 the climatic conditions prevailing in southern Missouri, by largely 

 increasing the vigor of growth over the average growth, several 

 limiting factors must be considcretl. In the case of pruning, such 

 large increase in vigor of growth can practically be secured only fol- 

 lowing winters or springs when the fruit buds or blossoms have been 

 killed, since this cutting back so severely in other years would cause 



