262 POMOLOGY 



232. Hardiness of different tissues. — As has been in- 

 dicated above, the different tissues of a fruit-tree vary in 

 hardiness, and they also change at different seasons of the 

 year. It has been shown that when the trees are in a young 

 growing condition, the cambium, young cortex, and sap-wood 

 cells are the tenderest while the pith in young twigs is the 

 first to be killed in mature trees followed by browning in the 

 sap-wood and the outer or old cells of the cortex. The nota- 

 ble point here is that cambium is most tender in the grow- 

 ing plant but relatively hardy when it is in winter condition. 

 This observation can sometimes be made with peach trees 

 after a severe winter when a cross-section of the limbs or 

 trunk will often look so brown or black that little or no hope 

 could be entertained for saving them. The cambium may 

 start into growth in the spring, however, and soon a new layer 

 of sap-wood is formed, which begins functioning, and re- 

 covery of the tree takes place. 



The fruit-buds of the peach are recorded to be about as 

 hardy as the cortex, cambium, and sap-wood of the twigs in 

 the latter part of summer, but during the winter they are 

 the most tender of all the tissues above ground with the pos- 

 sible exception of the pith cells. Usually the leaf-buds are 

 more hardy than the fruit-buds, but instances are on rec- 

 ord in which the leaf-buds and part of the sap-wood of the 

 peach have been killed or badly injured while a portion of 

 the fruit-buds have survived and produced, even in the ab- 

 sence of leaves.^ This is explained on the basis of lack of 

 maturity of the wood tissue, while the fruit-buds reached 

 maturity before the freezing occurred. 



233. Rest-period. — That perhaps all plants have a more 

 or less definite rest-period has been well established by a 

 number of writers, notably Klebs in Germany and Whit- 



1 Chandler, W. H. hoc. cit. p. 224. Paddock, W. Soc. Hort. Sci. 

 1918. p. 30. 



