286 PRINCIPLES OF BOTANY, ETC. 
circulation is possible. What we suspect at present, 
the labours of philosophers in some future period: 
will, it is to be hoped, establish as a truth, Per- 
haps this point, from which in vegetables the sap 
seems to ascend and descend, is only to be sought, 
where the parts above and below ground take their 
rise. 
The experiment mentioned before, (§ 259), to 
imvert a tree, and to change its roots into its top, 
and the reverse, has commonly been adduced as a 
proof of the ascent and descent of the sap. It has 
even been alleged, that by this means those channels 
which carry the sap upwards, are forced to send it 
downwards in their new position. But in making 
this objection, it seems to have been forgotten, that 
the sap must likewise circulate in the root, which 
not only sends it forth to the stem, but in summer 
grows itself larger, in the same proportion as the 
stem does: that Grew found the air vessels winding 
in the root in a different direction from the stem, 
(§ 235), and that we are not entitled to conclude 
that in an Inverted plant the same vessels must 
carry the sap in a reverse direction. It is not the. 
same thing to invert an animal, and to put it upon 
its head, and to invert a plant. The one will not 
remain long in this situation without being mate- 
rially hurt, whilst the other will not suffer from it. 
§ Dic 
What has been always adduced as another proof 
of the ascent and descent of the sap in plants, is the 
important, but altogether mistaken phenomenon, 
that 
