Chap. 2.] C'ourjt of tb'e Succus Prcpfius. :*$ 



in the fpring, when little or no flip had as yet en- 

 tered the plant, Dr. Hope made a number of inci- 

 fions of different altitudes, into the root and ftem of a 

 birch. As the fap rofe, it firft flowed from the fupe- 

 rior margin of the loweft incifion, and then, in regular 

 fucceffion, from the upper margins of the other inci- 

 fions, till at laft it reached the higheft. 2. If, in the 

 beginning of the bleeding feafon, before the fap is 

 found in the ftem or branches, an incifion is made in 

 the root of a vine, a confiderable flow of fap will 

 follow the wound, 3. The quantity of fap is very 

 generally proportioned to the humidity of the foil *. 



f When a portion of the bark and wood of the 

 pine is cut from the ftem, the Juccus propritis flows in 

 confiderable quantity both from the upper and under 

 margin of the incifion. Hence it occurred to bota- 

 niits, that this juice might have little or no motion, 

 and that its efflux from fuch an orifice might depend 

 entirely on its being freed from the preflure of the 

 bark and wood. But I cannot accede to this opinion i 

 for although, in the beginning, thejuccus proprius flows 

 from both margins of the incifion, in a little while, as 

 I have obferved, it is difcharged from the fuperior 



* It may ftill be afked, Why the fap flow* moil from the fupe- 

 perior margin of< each inciuou, iuppoliug. it to arife from the 

 roots ? The incifion, it is faid, hurts or deitroys the energy of the 

 fap-veffels fora confiderable way below, whence the lap is not- 

 propelled upwards againil its own weight, and the preflure of the 

 atmofphere now admitted. From the divided veflels, it pafics by 

 a lateral communication (for there aie fap-veflels in every direc- 

 tion) into thofe undivided, and when it has got above the incifion, 

 it again pafTes laterally into the divided veiTels; and falling down- 

 ward*, from its own gravity, a want of continuity of veflels, and 

 the diminifhed preffure of the stmofphere, i: flows from the fupe- 

 rior margin of the incifion. 



margin 



