674 < Microscopical Essays, 



in between the rays of the wood, meet in the center, and confti- 

 tute the pith. 



Of the Sap Vessels. 



The moll numerous and the largeft apertures are generally to 

 be found in the wood, which are perceived very diftinclly in a 

 tranfverfe lection, in which the ends of the veflels are feen as cut 

 through by the knife. The fcarlet oak of America is recommend- 

 ed 'as a proper objecl: for exhibiting them. If a fhort cylinder of 

 a three years branch of this oak, a little macerated, be hollowed 

 away with a chiflel, we fhall fee what a large portion of the wood 

 is occupied by thefe veiTels • they are thick and ftrong, and it is 

 eafy, with fome care and attention, to loofen feveral of them. 



If a number of thefe, thus feparated, be put into a vial of 

 rain water, and frequently (hook for feveral days, fome will at 

 length be found perfectly clean, thefe are then to be put into 

 fpirit of wine, and when that has been two or three times changed, 

 they will be in a condition to be viewed for undemanding their 

 ftrufture ; another method of preparation has been already fliewn 

 in page 162. 



Thefe are the veffels which have been called by fome writers 

 air, by others tracheal veflels. It is, however, to be remarked, 

 that moll of thole who have confidered them as air v fch, refer- 

 us to the tree while in a more herbaceous flate ; in tins cale they 

 fay, that we {hall find thefe parts filled with a line fpiral filament. 

 As thefe veflels are often to be found empty, they have been fup- 

 pofed to anfwer the purpofes of lungs to the plant. Malphigi 

 a affertSj, 



