PHYSIOLOGY. 333 
hue every year. The quick circulation of the sap 
only takes place in the young vascular circles; in 
the older ones the sap is carried along much slower, 
‘and they have their irritability greatly diminished. 
Life in every shrub or tree is seated only in the 
youngest rings of these vessels, which we now know 
under the name of the inner bark, (§ 280), and 
the plant must die when this is wounded. ‘Thus if 
a ligneous plant has performed its offices for a 
number of years, then the mnermost ring begins 
to be plugged up, and to become more and more 
impervious. Whence its neighbours no longer ob- 
tain any moisture from it. ‘They therefore begin to. 
move their sap slower, and the youngest vascular 
circle becomes gradually thinner and thinner. At 
last the.sap stops likewise in the following ligneous 
circles: the youngest vascular ring cannot form it- 
_self completely; few buds are now unfolded; the 
small number of leaves cannot prepare sufficient sap 
for the whole, and the common certain lot of or- 
ganized bodies, death, stops the machine entirely. 
§ 298. 
In herbaceous plants all the vessels of the stem 
become dry and hard in one twelvemonth, and as 
therefore they can no longer convey the sap, the 
stem decays at the end of the year. ‘Their root 
forms, as the stem of ligneous plants does, annually 
a new vascular circle, and it dies itself at last, when 
all those circles have become too ligneous. But 
such herbs, the roots of which are annually renewed, 
are of constant duration. ‘The old root dies, its 
fibres 
