REPORT ON THE PHILOSOPHY OF BOTANY. 49 



Origin of Organs. — There is no part of vegetable physiology 

 SO obscure as that which relates to the origin of organs. We 

 find a degree of simplicity that is perfectly astonishing in the 

 fundamental structure of the whole vegetable kingdom ; we 

 are able to prove by rigorous demonstration that every one of 

 the appendages of the axis is a modification of a leaf, to which 

 there is a constant tendency to revert; we see that in some cases 

 a part which usually performs one function assumes another, 

 as in the Alstromerias, whose leaves by a twist of their petiole 

 turn their under surface upwards : but we are entirely ignorant 

 of the causes to which these changes are owing. An impor- 

 tant step in elucidating the subject has been lately taken by 

 M. Mirbel, in his memoir upon the structure of Marchantia po~ 

 lymorplia. The young bulbs by which this plant is multiplied 

 are originally so homogeneous in structure, that there is no 

 apparent character in their organization to show which of their 

 faces is destined to become the upper surface, and which the 

 under. For the purpose of ascertaining whether there existed 

 any natural but invisible predisposition in the two faces to un- 

 dergo the changes which subsequently become so apparent, 

 and by means of which their respective functions are performed, 

 or whether the tendency is given by some cause posterior to 

 their first creation, the following experiments were instituted. 

 Five bulbs were sown upon powdered sandstone, and it was 

 found that the face which touched the sandstone produced 

 roots, and the opposite face formed stomata. It Avas, however, 

 possible that the five bulbs might have all accidentally 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 sto- 

 mata, and that the tendency was determined merely by the po- 

 sition in which the surfaces were placed. The next point to 

 ascertain was, whether the tendency once given could be after- 

 wards 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 originall)'^ 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 



1833. E 



