OF FOREST-TREES. 



123 



whilst your seedlings are very young, and till they come to be able to kill CHAP. VI I. 

 them with shade and over-dripping ; and then are you, for the obstinate, "'-"^^V^^ 

 to use the hoe, fork, and spade, to extirpate dog-grass, bear-bind, &c. 



And here, mentioning shade and dripping, though I cannot properly 

 speak of them as infirmities of trees, they are certainly the causes of their 



mustard ; to the nourishing grains, and to the deadly night-shade. The fibres of a root 

 are supposed to be simple capillary tubes ; but, upon a minute inspection, we discover 

 them to consist of the seven component parts of the plant. At their extremities we 

 observe a spongy kind of excrescence pierced with innumerable small holes. Through 

 these the nutritive juices of the earth are absorbed. When a plant has been pulled up, 

 it will be retarded in its growth, until Nature has renewed that spongy nipple. The bark 

 and leaves of a plant imbibe, at proper seasons, the moisture of the atmosphere. At other 

 times they perspire the superfluous nourishment. This opens to our view an extensive 

 prospect of the vegetable oeconomy. We have already seen that all the parts of a plant 

 are the same. They only differ in shape. The roots are formed sharp and pointed, to 

 make their passage easier through the earth. The leaves are made broad to catch the 

 moisture of the air with more readiness. When the root of a tree happens to be elevated, 

 instead of being retained within the earth, it assumes the appearance of a perfect plant, 

 with leaves and branches. Experiments shew us that a young tree may have its branches 

 placed in the earth, and its roots elevated in the air ; and in that inverted state it will 

 continue to live and grow. The air contains, especially during the summer months, all 

 the principles of vegetation: oil for the perfect food, water to dilute it, and salts to 

 assimilate it. These are greatly absorbed by the vessels of the leaves and bark, and con- 

 veyed to the innermost parts of the plant for its growth and fructification. When the air 

 happens to be cold and moist, this absorption takes place. When it is hot and dry, the 

 same vessels throw off the superfluous moisture by pei'spiration. In animals, the kidneys 

 and pores of the skin carry off the superfluity. The vegetable not having kidneys, perspires 

 more than the animal. Dr. Hales has demonstrated that this perspiration is considerable. 

 I shall here transcribe his statical experiments upon the sun-flower, for the benefit of those 

 who may not have an opportunity of examining the original. 



" July 3, 1724, in order to find out the quantity imbibed and perspired by the 

 " sun-flower, I took a garden-pot, (Plate 1. Fig. 5,) with a large sun-flower, a, 

 " 3 feet -f- J; high, which was purposely planted" in it when young : it was of the large 

 " annual kind. I covered the pot with a plate of thin milled lead, and cemented all the 

 " joints fast, so as no vapour could pass, but only air, through a small glass tube, b, nine 

 " inches long, which was fixed purposely near the stem of the plant, to make a free com- 

 " munication with the outward air, and that under the leaden plate. I cemented also 

 " another short glass tube, c, into the plate, two inches long, and one inch in diameter. 



Through this tube I watered the plant, and then stopped up also the holes, d, e, at the 

 " bottom of the pot with corks. I weighed this plant and pot morning and evening, for 

 " fifteen several days, from July 3, to August 8, after which I cut off the plant close to 



