572 APPENDIX C. 



of spiral or annular ducts, or cells partially united into such ducts, 

 colouring them deeply, and leaving the feebly-marked sheath of 

 prosenchyma, together with the surrounding watery cellular tissue, 

 perfectly uncoloured. 



The most conclusive evidence, however, is furnished by those 

 Cactacece in which the transition from succulent to dense tissue 

 takes place variably, according as local circumstances determine. 

 Opuntia yields good examples. If a piece of it including one of 

 the joints at which wood is beginning to form, be allowed to absorb 

 a coloured liquid, the liquid, running up the irregular bundles of 

 vessels and into many of their minute ramifications, is restricted to 

 these where they pass through the parenchyma forming the mass of 

 the stem ; but near the joints the hardened tissue around the vessels 

 is coloured. In one of these fleshy growths we get clear evidence 

 that the escape of the dye has no immediate dependence on the age 

 of the vessels, since, in parts of the stem that are alike in age, some 

 of the vessels retain their contents while others do not. Nay, we 

 even find that the younger vessels are more pervious than the older 

 ones, if round the younger ones there is a formation of wood. 



Thus, then, is confirmed the inference before drawn, that in ordi- 

 nary stems the staining of the wood by an ascending coloured liquid 

 is due, not to the passage of the coloured liquid up the substance of 

 the wood, but to the permeability of its ducts and such of its pitted 

 cells as are united into irregular canals. And the facts showing 

 this, at the same time indicate with tolerable clearness the process 

 by which wood is formed. What in these cases is seen to take place 

 with a dye, may be fairly presumed to take place with sap. Where 

 the dye exudes but slowly, we may infer that the sap exudes but 

 slowly ; and it is a fair inference that where the dye leaks rapidly out 

 of the vessels, the sap does the same. Inferring, thus, that where- 

 ever there is a considerable formation of wood there is a considerable 

 escape of the sap, we see in the one the result of the other. The 

 thickening of the prosenchyma is proportionate to the quantity of 

 nutritive liquid passing into it ; and this nutritive liquid passes 

 into it from the vessels, ducts, and irregular canals it surrounds. 



But an objection is made to such experiments as the foregoing, 

 and to all the inferences drawn from them. It is said that portions 

 of plants cut off and thus treated, have their physiological actions 

 arrested, or so changed as may render the results misleading ; and it 

 is said that when detached shoots and leaves have their cut ends 

 placed in solutions, the open mouths of their vessels and ducts are 

 directly presented with the liquids to be absorbed, which does not 

 happen in their natural states. Further, making these objections 

 look serious, it is alleged that when solutions are absorbed through 

 the roots, quite different results are obtained : the absorbed matters 

 are found in the tissues and not in the vessels. Clearly, were the ex- 



