306 PRINCIPLES OF BOTANY, ETC. 
knot of the petiole. Now, though this quantity of 
sap is very inconsiderable, yet its motion naturally 
will cause some sort of concussion, which perhaps 
is alone sufficient to make the leaves finally fall off. 
In the oak tree the leaves cannot fall off in au- 
tumn, as the vascular fibre of this tree is very tough, 
and on this account the connexion between the knot 
of the petiole and the stem is not broken. In the 
Robinia pseudacacia the small and tender petioles of 
its leaves first get closed up by the sap, and separate 
of course earlier from the common petiole, which is 
still succulent enough to remain a short time, but 
soon, as without the leaves it cannot subsist, has the 
same fate. It depends therefore entirely on the na- 
ture of the leaf, how long it is to remain on the 
stem, not on the weather. Besides, the natural or- 
ganization must be attended to, as it has a powerful 
influence. 
§ 284. 
The growth of the plant ends with the evolution 
of the flower. When a plant has acquired a certain 
degree of firmness, (which, as they are so mult- 
farious, does not happen in each at the same time, 
or at the same age), it then becomes capable of pro- 
pagating its own species, and that part which we 
know under the name of the flower, is now formed. 
Its beginning, or the quickly expected final evolu- 
tion, in herbaceous plants, may generally be observed 
from the circumstance, that the minute scaly leaves 
erow gradually less, till the smaller and more deli- 
cate parts of the flower are at last unfolded. Goethe 
° 
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