THE FORMATION OF WOOD IN PLANTS. 409 
Cereus, and in two species of Mammillaria, which I have tried, I have foünd this so. 
Mammillaria gracilis may be named as exemplifying the relation under its extreme form. 
Into one of these small spheroidal masses, the dye ascends through the large bundles 
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 sur- 
rounding watery cellular tissue, perfeetly uncoloured. 
The most conclusive evidence, however, is furnished by those Cactacee in which the 
transition from succulent to dense tissue takes place variably, according as local circum- 
stances 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 ordinary stems the stain- 
ing of the wood by an ascending coloured liquid is due, not to the passage of the co- 
loured liquid up the substance of the wood, but to the permeability of its duets and 
such of its pitted cells as are united into irregular canals. And the facts showing this, 
at the same 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 wherever 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 irre- 
Sular 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 ob- 
Jections 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 
hot in the vessels. Clearly, were the experiments yielding these adverse results con- 
ducted in unobjectionable ways, the conclusion implied by them would negative the 
Conclusions above drawn. But these experiments are no less objectionable than those 
^ 
to which they are opposed. Such mineral matters as salts of iron, solutions of which 
