CHAP. 11. 
LEAVES. 
101 
fallen upon the face wliich was predisposed to emit roots; 
other experiments of the same kind were therefore tried, first 
with eighty, and afterwards with hundreds of little bulbs, — 
and the result was the same as with the five. This proved 
that either face was originally adapted for producing either 
roots or stomates, and that the tendency was determined 
merely by the position in which the surfaces were placed. 
The next point to- ascertain was, whether the tendency once 
o'iven could be afterwards altered. Some litde bulbs, that had 
been growing for twenty-four hours only, had emitted roots ; 
they were turned, so that the upper surface touched the soil, 
and the under was exposed to light. In twenty-four hours 
more the two faces had both produced roots : that which had 
originally been the under surface went on pushing out new 
roots ; that which had originally been the upper surface had 
also produced roots : but in a few days the sides of the young 
plants began to rise from the soil, became erect, turned over, 
and finally recovered in this way their original position, and 
the face which had originally been the uppermost immediately 
became covered with stomates. It, therefore, appears that, 
the impulse once given, the predisposition to assume particular 
appearances or functions is absolutely fixed, and will not 
change in the ordinary course of nature. This is a fact of 
very high interest for those who are occupied with researches 
into the causes of what is called vegetable metamorphosis. 
The parenchyma is, if casually examined, or even if viewed 
in slices of too great thickness, apparently composed of heaps 
of small green bladders, arranged with little order or regu- 
larity; but, if very thin slices are taken and viewed with a 
high magnifying power, it will be seen that nothing can be 
more perfect than the plan upon which the whole structure is 
contrived, and that, instead of disorder, the most wise order 
pervades the whole. Upon this subject I extract the words 
of Adolphe Brongniart : — " There exists beneath the upper 
cuticle two or three layers of oblong blunt vesicles, placed per- 
pendicular to the surface of the leaf, and generally much less 
in diameter than the bladders of the cuticle ; so that they are 
easily seen through it. These vesicles, which appear specially 
destined to give solidity to the parenchyma of the leaf, hav^ 
ii ;3 
