dajj and cold by night, are alternately dilated and 

 contracted. 



Is not then the motion of the sap hi plants (like that 

 of the blood in animals) produced chiefly by the action 

 of the air? A1J plants have the two orders of vessels 

 1, Those which convey the nutritious juices; 2. Air- 

 vessels, hollow tubes, within which all the other ves- 

 sels arc contained. Now the least heat rarefies the 

 air in these air-vessel s, thereby dilating them, arid so 

 causing a perpetual spring, which promotes the circu- 

 lation of the juices, For by the expansion of the air- 

 Tessels, the sap-vessels are pressed, and the sap con- 

 tinually propelled. By the same propulsion it is com- 

 minuted more and more, and so fitted to enter finer 

 and finer vessels, while the thicker partis deposited in 

 the lateral cells of the bark, to defend the plant from 

 cold and other injuries. 



Thus is every plant acted on by heat in the daytime, 

 especially in summer; the sap protruded, then eva. 

 cuated, and then exhausted. In the night the air- 

 vessels being contracted by the cold, the sap-vessels 

 are relaxed, and disposed to receive fresh food for the 

 next day's digestion. And thus plants do, as it were, 

 eat and drink during the night season. 



The vessels themselves consist of mere earth cement- 

 ed by oil and water, which being exhausted by fire, 

 air, or age, the plant returns to its earth. Thus in 

 plants burnt by the fiercest fire, the matter of the ves- 

 sels is left entire, which consequently is neither water, 

 air, salt, nor sulphur, but earth alone. The sap con. 

 sists of some saline parts, others derived from air, rain, 

 and putrified plants or animals. Consequently in 

 plants are contained salts, oils, water, earth, and pro- 

 bably all metals too. In fact, the ashes of all vege- 

 tables yield something, which the loadstone attracts. 



There is a considerable difference as to the time 

 when different plants revive after the winter. No 

 sooner does the sun begin to warm the earth, than the 

 vernal flowers appear, and the trees one alter another 

 open their buds ; and clothe themselves with leaves* 



