PHYSIOLOGY. 285 
found himself induced by Van Marum’s observa- 
tions, to change it, and to admit the irritability of 
the vessels, as the sole cause of the circulation of the 
sap in them. 
If we now contemplate the vegetable world with 
attention, and accurately observe this phenomenon 
in it, we will no longer doubt, provided a conelu- 
fion from analogy be allowed, that in plants as well 
as in animals, a real circulation of the sap takes 
place, not a mere ascent and descent of it. Still, 
however, nobody has yet proved it, and few indeed 
have even with Malpighi and others ventured to ad- 
mit it. But is it possible, that through a mere ascent 
and descent of the sap, the leafless tree is able to 
resist the cold, if there be not a circulation of the 
sap? A stoppage of the motion of the sap, or a con- 
stant descent of it during cold, certainly cannot be 
maintained; it is even contradicted by experience, 
If we admit the first, then the sap of a tree would 
congeal without injury during winter. Now we 
know, which happens especially with delicate exotic 
plants, that by a sudden invasion of intense cold 
the sap congeals, and the plant, at least most of its 
parts, are lost. If, on the contrary, we believe that 
the sap in winter is constantly descending, whence 
proceeds all the moisture during this long period, 
especially as the temperature is so low that even a 
delicate leaf cannot subsist? There must be a cir- 
eulation, of whatever nature it be. 
We have not yet found in vegetables one point, 
like the heart of animals, from which the motion of 
the sap commences. But it does not follow, that no 
circu: 
