24 



vessel is a closed sac or tube, that when any vegetable tissue be- 

 comes fossilized, the silica occupying their interior aud their inter- 

 spaces is, in fact, in detached pieces, each being separated from the 

 adjoining cell or vessel by the intervening walls of the tissue. If 

 fossilization went no further, and there is reason to believe that in 

 some cases it does not, the mass could easily be broken down by 

 slight force, and each original fibre detached from its neighbour on 

 account of the vegetable matter, after long maceration in the silicify- 

 ing fluid, being almost decomposed. But frequently the process 

 goes further; and as we know how readily vegetable membrane 

 transmits liquids through its substance, it can be easily imagined how 

 silica held in solution in the water would pervade it, and the inter- 

 cellular spaces and the interior of the woody fibres would be cemented 

 together into one mass of silica. 



" The reason why some woods break down more easily than others 

 after being fossilized, I have not yet been able to determine; but it 

 is certain that coniferous woods are found to be the most frequent 

 examples in which the tissue is not cemented, and I imagine that in 

 those woods there is great power of resisting decomposition when im- 

 mersed in water, or there exists little or no silica as an organized 

 part of their skeleton, so that no points in the membrane for the com- 

 mencement of deposits are offered ; whereas, where silica does exist, 

 the molecules form the first centres, and the whole become cemented 

 together. 



" It is thus, I am induced to believe, that silicification in the above 

 instances proceeded so far as to fill the fibres, vessels and cells, and 

 the spaces on their exterior ; but as the vegetable membrane was in- 

 terposed, the complete cohesion of the parts was prevented, and 

 consequently they are now capable of being separated, and the frus- 

 tules of silica when examined prove to be casts of the interior of the 

 tissues and of the interspaces external to them, thus appearing to 

 offer the most satisfactory evidence respecting the nature of the or- 

 gans in question." — p. 149. 



IX. An Enumeration of the Plants of the Galapagos Archipelago, 

 with Descriptions of those which are New. By Joseph Dalton 

 Hooker, Esq., M.D., F.L.S., &c. 



X. On the Vegetation of the Galapagos Archipelago as compared 

 with that of some other Tropical Islands and of the Continent of 

 America. By Joseph Dalton Hooker, Esq., M.D., F.R.S., F.L.S., &c. 



The materials for constructing this Flora have been for the most 

 part furnished by Charles Darwin, Esq., who drew the attention of the 



