uo 



FURTHER OBSERVATIONS 



It is easy to stop a gentle oozing from an. old 

 wound of a Vine, by applying soft clay, putty, or 



from the pores of their leaves and bark. At that time the juices 

 are highly rarefied ; and' the diameters of the tracheae, or air- 

 vessels, are enlarged so as to press upon and straiten the vessels 

 that carry the sap. In consequence of which, their juices, not 

 being able to escape by the roots, are pressed upward, where 

 there is the least resistance, and perspire of the excrementitious 

 parts by the leaves and top-branches in theform of vapour. When 

 the solar heat declines, the tracheae are contracted. The sap- 

 vessels are enlarged, and the sap sinks down in the manner of 

 the spirits in a thermometer. In consequence of this change, 

 the capillary vessels of the leaves 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 contained 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 economy, so far as it regards the 

 management of fruit-trees, and, probably, may be extended 

 through out the]whole system of gardening, planting, andfarming. 



" It may be objected, that trees fixed to the wall do, not- 

 withstanding, carry their sap to the extreme branches ; but it 

 should be considered, that the warmth of their situation, assisted 



