CIRCULATION AND FORMATION OF WOOD IN PLANTS. 595 



mechanical principles that the strains do act in such ways as to aid 

 the increase of the strengths. How a like correspondence between 

 two a priori arguments holds in the case of the circulation, needs 

 not to be shown in detail. It will suffice to remind the reader 

 that while the raising of sap to heights beyond the limit of cap- 

 illarity implies some force to effect it, we have in the osmotic 

 distention and the intermittent compressions caused by transverse 

 strains, forces which, under the conditions, cannot but tend to 

 effect it ; and similarly with the requirement for a downward 

 current, and the production of a downward current. 



Among the inductive proofs we find a kindred agreement. Diffe- 

 rent individuals of the same species, and different parts of the same 

 individual, do strengthen in different degrees ; and there is a clearly 

 traceable connexion between their strengthenings and the intermit- 

 tent strains they are exposed to. This evidence, derived from contrasts 

 between growths on the same plant or on plants of the same type, is 

 enforced by evidence derived from contrasts between plants of diffe- 

 rent types. The deficiency of woody tissue which we see in plants 

 called succulent, is accompanied by a bulkiness of the parts which 

 prevents any considerable oscillations; and this character is also 

 habitually accompanied by a dwarfed growth. When, leaving these 

 relations as displayed externally, we examine them internally, we 

 find the facts uniting to show, by their agreements and differences, 

 that between the compression of the sap-canals and the production 

 of wood there is a direct relation. We have the facts, that in each 

 plant, and in every new part of each plant, the formation of sap- 

 canals precedes the formation of wood ; that the deposit of woody 

 matter, when it begins, takes place around these sap-canals, and 

 afterwards around the new sap-canals successively developed ; that 

 this formation of wood around the sap-canals takes place where the 

 coats of the canals are demonstrably permeable, and that the amount 

 of wood-formation is proportionate to the permeability. And then 

 that the permeability and extravasation of sap occur wherever, in 

 the individual or in the type, there are intermittent compressions, 

 is proved alike by ordinary cases and by exceptional cases. In 

 the one class of cases we see that the deposit of wood round the 

 vessels begins to take place when they come into positions that 

 subject them to intermittent compressions, while it ceases when 

 they become shielded from compressions. And in the other class 

 of cases, where, from the beginning, the vessels are shielded from 

 compression by surrounding fleshy tissue, there is a permanent 

 absence of wood-formation. 



To which complete agreement between the deductive and induc- 

 tive inferences has to be added the direct proof supplied by experi- 

 ments. It is put beyond doubt by experiment that the liquids ab- 

 sorbed by plants are distributed to their different parts through their 



