564 



jiples that the strains do act in such ways as to aid the increase of 

 fche strengths. How a like correspondence between two ci priori 

 arguments holds in the case of the ch'culation, needs not to be shown 

 m detail. It will suflBce to remind the reader that while the 

 raising of sap to heights beyond the limit of capillarity impUes some 

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

 tent compressions caused by transverse strains, forces which, under 

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

 quirement for a downward cmTent, and the production of a down- 

 ward current. 



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

 rent individu^'^ls of the same species, and different parts of the same 

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

 traceable comiexion between vbeir 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, leavmg these rela- 

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

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

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

 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-canals takes place where thf 

 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, Id 

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

 proved ahke by oi-dinary 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 

 Ihem to intermittent compressions, «hile 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 sur- 

 rounding fiesny tissue, there is a permanent absence of wood-forma- 

 tion. 



To which complete agreement between the deductive and iuduc- 

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

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

 sorbed by plants are di.^tril^uted to their different parts through theij 



