HEAT AND MOISTURE. 123 



The great importance of heat in the economy of plants and for the 

 purposes of gardening should be attentively regarded. The tempera- 

 ture of springs affords no satisfactory evidence of that of the surface 

 of the ground, as they mostly come from a distance below, where the 

 temperature is equal at all seasons. The changes within the soil 

 through which roots penetrate should be ascertained monthly, so as to 

 compare the temperature with the state of vegetation. The greatest 

 difference between the mean temperature of the earth and atmosphere 

 appears to be in October, when at a foot below the surface, the former 

 is from one to one and a half degrees above that of the latter. It ap- 

 pears also that in the spring when vegetation commences, the mean 

 temperature of the earth is higher than that of the atmosphere by 

 from one to two degrees and that this difference remains thus higher 

 in autumn ; by which the condensation of the tissue and secretions of 

 perennial plants against the approach of cold weather is effected. The 

 average difference of temperature between 1 and 2 feet below the 

 surface is as 48.87 to 50. J 5, the atmosphere averaging 48.26. The 

 comparative difference in hot latitudes is often very great, as in the 

 tropics, where the temperature, just below the surface, is often from 

 126 J to 134', while that of the atmosphere is 84.5; in France 

 118-122', atmosphere 91.5" ; C. Good Hope, 59 and in the shade 119, 

 atmosphere 98. The mean temperature of the state of New York, 

 for 14 years has been 44.31", Fahr. ; the lowest mean for a single 

 year was 44.11, and the highest 49.99. The mean temperature near 

 this city during the coldest month (January 1839) was 28.89, and of 

 the warmest (July) 70.69"- At Albany the mean of January was 

 23.38 ', and of July 72.38 '. It is calculated, on the whole, that the 

 mean difference of temperature between the earth and atmosphere, 

 as before noticed, is uniformly from 1 to 2 1 . 



Moisture has not only an important effect on vegetables, but is an 

 essential element of their substance. Those having a large or spongy 

 cellular tissue necessarily absorb a large amount of water, and also 

 those with soft and broad leaves, provided with many cortical pores, 

 and likewise those with long and numerous roots. Such cannot live 

 without an abundant supply of moisture. Nor can those live in moist 

 and damp places which have a compact tissue, with small leaves having 

 few pores and clothed with hair, nor yet those depositing much oily 

 or resinous matter. The more water abounds with substances which 

 obstruct its passages, the slower will be its absorption. These sub- 

 stances more or less effect the topographical distribution of plants. 

 They consist mostly of carbonic acid, atmospheric air, alkaline earths 

 and animal and vegetable substances. Plants containing much car- 

 bon, such as those producing hard wood, avoid water that is mostly 

 free from carbonic acid gas, but the fungi, as the mosses, etc., prefer 

 such situations ; so also those requiring other materials for their com- 



