324 EVILS OF MOSS AND LICHEN. 



ceeded, it becomes an evil. For, though their tend- 

 ency be to mitigate the severity of both heat and 

 cold, they do in all cases produce cold by making 

 the total evaporation greater than it otherwise would 

 be. They take off the extremes of evaporation dur- 

 ing the great heats, but they also occasion evapora- 

 tion at times when otherwise there would be little 

 or none, and thus they keep a moist atmosphere all 

 the year round, and so where they abound the cli- 

 mate is less healthy. It also rains much more fre- 

 quently, because that air, being always nearly satu- 

 rated with moisture, is of course disposed to part with 

 that moisture in the state of rain, much more readily, 

 that is, with much less atmospheric action, than 

 when the degree of saturation is less. Consequently 

 they are injurious to cultivated grounds. To the 

 annual crops they, indeed, do small harm, as they 

 attain but little size, and are under the shade of 

 these. But on grass lands they are much more de- 

 structive ; and would in time change a good soil 

 and climate into the opposite. The finest grasses, 

 though they thrive well with occasional irrigations, 

 decay when they are too moist, as they always are 

 in old pastures that have got mossed ; and if the sur- 

 face had little drainage, the mosses would, in time, 

 dislodge all the grasses, and produce a surface not 

 well adapted for any kind of culture. When land 

 has once come to that state, the only, or at least the 

 effectual, means of arresting the mischief, are the 

 spreading of alkaline substances, trenching, or paring 

 off and burning the sod. 



But it would be endless to enumerate even the 

 trains of speculation and inquiry that present them- 

 selves to any one who studies vegetables, in their 

 connexion and succession, however narrow the field 

 of observation may be. A step taken anywhere that 

 there are plants furnishes a study; and that walk 

 which does not afford reflection for a week must 

 be very short, as well as over a place comparatively 



