186 DISEASES OF THE PINE. 



is greater than in a cold one. If, tlierefore, the temperature of the soil 

 is too low, the absorption of water by the roots is not sufficient to sup- 

 ply the loss of evaporation by the leaves, and the plants may wither 

 and die, althougli the soil contains sufficient moisture. 



Experiments made by Professor Sachs have shown that the roots of 

 tobacco and cabbage in a moist soil, the temperature of which had fallen 

 below 5° centigrade (41° F.), did not absorb water sufficiently to supply 

 the loss by evaporation, in consequence of which the plant wilted.^ 



By our observations, extending through a series of years, we find 

 that in the months of March and April (according to location, condition 

 of soil, and weather) the temperature of the earth, even at a depth of four 

 feet, often barely reaches 4° R. (41° F.), while the temperature of the 

 air is frequently from 15° to 18^ K. {G(j<^ to 72° F.) higher. On account 

 of the cold in the earth prevailing at this season, the activity of the 

 roots of plants is weak, and the absorption of water proportionally 

 small. In plants which at this season have organs of transpiration, 

 which is the case with evergreens, the same results may occur as with 

 the cabbage and tobacco in a degree proportionate to their exposure to 

 direct solar light. All of these results point to the fact that the " Kiefer- 

 Shutf^ is not a disease caused by frost, but a drying up and withering 

 of the pine leaves occasioned by the high temperature of the clear days 

 in the early months of spring, and by too great insolation of the un- 

 shaded plants in the clearings. 



This wilting differs from that which occurs during the summer in 

 this, that the insufficiency of water in the plant, occasioned by trans- 

 piration, is not caused by a lack of moisture in the soil, but by an insuf- 

 ficient activity in the roots, in consequence of which the leaves must 

 die. The greater the difi'ereuce between the temperature of the air in 

 the direct light of the sun, as compared with that of the earth, by so 

 much more frequently and destructively will this disease appear. Every 

 circumstance tending to increase the temperature of the earth, or to 

 reduce that of the air, or to lessen the intensity of the sun's light, 

 and consequently to retard the evaporation of plants, would therefore 

 operate against the appearance of this disease. 



The temperature of the earth during the early spring months depends 

 on the severity or mildness of the preceding winter. In the spring 

 following a winter with an abundance of snow, the soil is warmer than 

 after one in which there has been but little snow. Wet soils under cir- 

 cumstances otherwise alike, will be colder than those that are dry. 

 Sandy soils cool off by radiation more quickly than clay soils, during the 

 night. The soil in seed-beds which have been covered with straw during 

 the winter, is warmer than that which has been left uncovered. Warm 

 rains contribute largely to the raising of the temperature of the earth, 

 while by frosts in clear nights the temperature of the earth is consider- 

 ably lowered to a depth of six inches. It is from this reason chiefly 

 that late frosts have any relation to this disease. 



The temperature of the air, and the intensity of light, of course reach 

 a greater degree in clear days in the spring time than under a clouded 

 sky, and as evaporation 4n plants is hastened by the direct light of the 

 sun, the reason becomes apparent why this malady appears oftener in 

 the spring, in clear days, and in places most exposed to these changes, 

 as, for example, those with an eastern or southern exposure — on -bare 



'A discussion of the question of the movement of water in plants is given in detail 

 at pp. 598-614 (English edition) of Sachs's Text Book of Botany, Morphological and Phys- 

 iological (1875). See also Botanical Gazette, 1860, p. 124. 



