564 



siples that the strains do act in such ways as to aid the increase o! 

 the strengths. How a like correspondence between two & 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 capillarity implies some 

 force to effect it, we have in the osmotic distention and the intermit- 

 tent compressions caused by transverse strains, forces which, under 

 (lie conditions, cannot but tend to effect it ; and similarly with the re- 

 i(uirement for a downward current, and the production of a down- 

 ward 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 vheir strengthenings and the intermittent 

 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 habi- 

 tually accompanied by a dwarfed growth. When, leaving these rela- 

 tions as displayed externally, we examine them internally, we find 

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

 betv/een 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 wood} 

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

 afterwards around the new sap-canals successively developed; thai 

 this formation of wood around the sap-panals takes place where thf 

 coats of the canals are demonstrably permeable, and that tlie amouni 

 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 

 beo-ins 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, 

 froui the beginning, the vessels are shielded from compression by sur- 

 roundmg fleshy tissue, there is a permanent absence of wood-forma- 

 tion. 



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 theii 



