272 Of the Pith of Press. 
the wood and the inner bark, and leading down to the 
covering of the root, which he alfo calls venal veffels, 
and fuppofes them to contain the liquid fap found in 
plants in the fpring. 
Fourth, the bark being of a fpongy texture, which by 
many little firings communicates with the pith. 
Fifth, the pith, or pedlen, which confifls of little 
tranfparent globules, chained together fomewhat like the 
bubbles that compofe the froth of liquor. 
Malpighi was the firfl who obferved, that vegetables 
confifl of two forts of veffels. i.Thofe abovementioned, 
which receive and convey the alimental juices. 2.Tracheae, 
or air veffels, which are long hollow pipes, wherein air 
is continually received and expelled, i. e. within which 
tracheae he fhews all the former feries of veffels are 
contained. 
Hence it follows, that the heat of a year, nay of a 
day, of a fingle hour, or minute, mufl have an effedl on 
the air, included in thefe tracheae, i. e. it mufl rarify it, 
and confequently dilate the tracheae; whence alfo mufl 
arife a perpetual fpring or force of action to promote the 
circulation in plants. 
For by the expanfion of the tracheae, the veffels 
containing the juices are preffed; and by that means 
the contained juice is continually propelled, and fo 
adcellerated; by which fame propulfion the juice is 
continually comminuted and rendered more and more 
fubtle, and fo enabled to enter veffels flill finer and 
f 
finer j the thickefl part of it being at the fame time 
fecreted and depofited into the lateral cells, or loculi of 
the bark, to defend the plant from cold and other ex** 
ternal injuries. 
The juice being thus conveyed from the root, to the 
remote branches, and even to the flower j and having in 
everv 
