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bility of the vascular system in plants appear to 
depend upon entirely different causes. 
In crystallization, or the regular arrangement of 
inorganic substances, there is a constant increase 
of matter from the attraction and juxtaposition of 
like parts or molecules. In vegetation a germ ex- 
pands by the assimilation of a variety of new ali- 
ments, and by powers entirely different from those 
of common inorganic matter ; but there seems to 
be no system of nerves as in animals, which is 
essential to irritability. We know so little of the 
refined powers and properties of matter, that we 
can give little more than vague hypotheses as to 
the cause of the movement of the fluids in the 
vegetable cells or tubes, yet it is impossible not to 
allow common material agents a much greater share 
in producing this phenomenon, than they exercise 
in animal life. 
Whoever will peruse any considerable part of 
the Vegetable Statics of Hales must receive a 
deep impression of the dependence of the motion 
of the sap upon physical causes. In the same 
tree, this sagacious person observed, that in a cold 
cloudy morning, when no sap ascended, a sudden 
change was produced by a gleam of sunshine, of 
half an hour ; and a vigorous motion of the fluid. 
The alteration of the wind from south to the north 
immediately checked the effect. On the coming 
on of a cold afternoon after a hot day, the sap 
that had been rising began to fall. A warm shower 
and a sleet storm produced opposite effects. 
Many of his observations likewise show, that the 
