CHAP. II. 



LEAVES. 101 



fallen upon the face which 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 

 given could be afterwards altered. Some little 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 

 vei'y 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 vipper 

 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 speciallv 

 destined to give solidity to the parenchyma of the leaf, have 



H 3 



