14 MANUAL OF TREE DISEASES 



that plant. The stem- and root-tissues develop added resist- 

 ance to low temperatures if they mature properly in the autumn. 

 The exact nature of this maturing process is not understood, 

 but growth must cease and the tissues must pass slowly into 

 the dormant stage. If the plant passes rapidly from the active 

 vegetative condition into dormancy, the tissues are killed at a 

 much higher temperature. The process of maturing in small 

 seedlings is relatively much more rapid than in large trees. The 

 roots are more liable to injury in the case of seedlings because 

 they cease growth last and mature later than the aerial parts. 

 They are also subject to injury because the feeding root-tips and 

 tenderest rootlets are close to the surface of the soil. For a 

 further discussion of freezing-to-death in older trees, see page 47. 

 In the case of early or fall frosts, a sudden drop of temperature 

 below 32° Fahr. may cause injury. Broad-leaf trees are more 

 susceptible to early frost-injury than conifers. Injury to 

 seedlings may result in the spring from late frosts. Tempera- 

 tures a little above 32° Fahr. may cause serious damage ow- 

 ing to the extreme susceptibility of the new tissue. 



Control. 



High ground should be selected for the site for the seed- and 

 transplant-beds in regions where low temperatures are common. 

 At the same time, where sandy soil and danger from sun-scorch 

 may be encountered, a site which will avoid both high and low 

 spots should be selected. Early in the autumn only slight 

 care should be given the beds since the trees should be allowed 

 to cease growth and enter the dormant condition. Loose 

 mulches advised under winter-drying (see page 11) will pro- 

 tect the plants to some degree against freezing-to-death, es- 

 pecially by keeping the temperature more uniform and de- 

 creasing the rapidity of temperature changes. The mulch 

 should be allowed to remain on the beds as long as is safe in 

 the spring in order to retard the beginning of growth. 



