OF rOREST-THEES. 



129 



perhaps, more advisable to draw it out by a deep incision, and to depend CHAP. 

 upon a new supply, than, upon confidence of correcting this evil quality ^-'"'^ 

 by other medications, to let it perish. Other causes of their sickness, not 

 always taken, notice of, proceed from too liberal refreshments and over- 

 watering in dry and scorching seasons, especially in nurseries : the water 

 should, therefore, be fitly qualified, neither brackish, bitter, stagnant, nor 



and top-branches become empty. Being surrounded with the humid vapours of the evening, 

 they fill themselves from the known laws of attraction, and send down the new-acquired 

 juices to be mixed with those that are more elaborated. As soon as the sun has altered the 

 temperature of the air, the tracheae become again distended, and the sap-vessels are 

 straitened. The same cause always produces the same effect: and this alternate ascent 

 and descent, through the same system of vessels, continues as long as the plant survives. — 

 The irregular motion of the stem and branches, is another cause that contributes to the 

 ascent of the sap. Every time that these parts are acted upon by the air, they are made 

 to assume a variety' of angles, whereby the sap-vessels are suddenly straitened. The con- 

 tained juices consequently receive reiterated impulses, similar to what happens to the 

 blood of animals from the contraction of the heart. This observation may assist us in 

 investigating the vegetable ceconomy, so far as it regards the management of fruit-trees, 

 and probably may be extended throughout the whole system of gardening, planting, and 

 farming. It may be objected, that trees fixed to the wall do, notwithstanding, carry their 

 sap to the extreme branches ; but it should be considered that the warmth of their 

 situation, assisted by the horizontal direction of their branches, is fully sufficient to propel 

 the sap, without the undulatory motion that I have mentioned. 



I beg leave to remark, that these observations are only intended to convey a general 

 idea of the motion of the sap. It varies according to the temperature of the weather. — 

 The air is seldom one moment alike. The sap must, therefore, sometimes move quick, 

 sometimes slow. It may rise and fall many times in a day. Sudden heats push it upward, 



sudden colds make it fall. Thus the juices are blended, and the secretions forwarded. 



The manner that the nutritive juices of the earth and atmosphere are conveyed into the sap- 

 vessels, remains to be described. And this makes a necessary part of our present argu- 

 ment, as it may assist us in finding out, and explaining, the diseases of plants from the 

 variations of the weather. The outer bark which covers every external part of a 



vegetable, as well below as above the surface, is full of perspiratory or absorbent holes. 



The vessels of this bark being endowed with the power inherent in capillary tubes, draw 

 up the moisture that is applied to their surface. From them it is committed to the vessels 

 of the inner bark. After receiving some degree of melioration, the sap is delivered to the 

 blea. From the blea, it passes, by anastomosing canals, to the vascular series: from 

 thence to the wood, or flesh, where it receives its last concoction. The nutritive particles, 

 being separated by the mechanism of these numerous canals, are applied towards the 

 fructification and increase of the plant ; while the watery and excrementitious parts are 

 carried expeditiously to the leaves, where they are perspired off in the form of vapour.— 



