690 PROTECTION AGAINST DISEASES. 



meteorological stations, was led to adopt this view of the 

 matter. His theory is, shortly, as follows : — The young Scots 

 pine plants, owing to the frequently high atmospheric tem- 

 perature in March and April (66° to 77° F. in the shade), 

 are on sunny days compelled to transpire freely. Although 

 the soil is wet enough to replace the loss of water hy trans- 

 piration, the action of the roots is restricted l)y the cold soil, 

 the temperature of which may be only 40^ F., or less, down 

 to a depth of 4 feet. Hence the little plants wilt and the 

 needles dry up and die. This is not due, as in dry summers, 

 to the absence of moisture in the soil, but to the inability of 

 the roots to absorb water in the cold ground, and therefore an 

 abnormal drying-up of the needles ensues. 



This theory will not explain needle-cast in autumn, when 

 the soil is warmer than the air, but when the needles are cast 

 in the spring, it is in comj^lete accordance with the observa- 

 tions recorded on pp. 686 — 7, under the headings " Locality " 

 and "Weatlrer." Sandy soils cool down at night to lower tem- 

 peratures, under similar conditions, than clays, and wet soils 

 become colder than dry soils. Insolation is greatest on bare 

 southern aspects. 



5. Damacje done. 



As a rule, needle-cast is not fatal to the plants, and those 

 which have been attacked may recover, provided their terminal 

 buds are still uninjured. Naturally, however, the injured plants 

 languish for some time and are very liable to be attacked by 

 insects. If, however, the disease recurs, and the terminal buds 

 of the plants suffer, they have no chance of recovering. 



6. Treatment. 



As proper treatment of the disease will depend on its 

 origin, the present section will be divided into headings 

 according as the disease is due to a fungus, frost or the drying 

 up of the plants. The method of contending with the attacks 

 of the fungus has been already described on p. 465, and only 

 the two latter causes will be dealt with here. 

 a. Frost. 



i. Regenerate Scots pine woods under a shelterwood. 

 Avoid large felling-areas in clearing Scots pine woods, and 



