428 . MR. SPENCER ON CIRCULATION AND 
their surfaces, and increase the exudation of nutritive matters from them, and must do 
this actively in proportion as the bends are great and frequent; so that while, on the 
one hand, it is a necessary deduction that, if the parts of each plant are to be severally 
strengthened according to their several strains, there must be some direct connexion 
between strains and strengths, it is, on the other hand, a necessary deduction from me- 
chanical principles that the strains do act in such ways as to aid the increase of the 
strengths. How a like correspondence between two à 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 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. Different 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 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 different types. The defieieney 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 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 forma- 
tion of wood around the sap-canals takes place where the coats of the canals are demon- 
strably permeable, and that the amount of wood-formation is proportionate to the pe 
meability. And then that the permeability and extravasation of sap occur wherever, D 
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 - 
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 bom 
shielded from compressions. And in the other class of cases, where, from the be 
ginning, the vessels are shielded from compression by surrounding fleshy tissue, there # 
a permanent absence of wood-formation. 
To which complete agreement between the deductive and inductive inferences has si 
be added the direct proof supplied by experiments. It is put beyond doubt by expe" 
ome that the liquids absorbed by plants are distributed to their different parts t i 
uu mE first by the spiral or allied vessels originally developed, and Je p 
-P'aced ducts formed later. By experiment it is demonstrated that the ™ 
