610 PHYSIOLOGY. 



be carefully dissected off from a young vine-shoot, so as to leave the pith 

 exposed ; and it will be seen that, while the bark and wood become con- 

 tracted (shorten, in other words), the pith lengthens. A similar contrac- 

 tion ensues, but in a transverse direction, when a ring of bark is cut off 

 from a shoot. Try to fit it on to the stem from which it was taken, 

 and it will be found impossible to replace it accurately. Dr. Bird deci- 

 sively proved that the divergence in question was due to varying degrees 

 of tension, these variations being dependent in his experiments on the 

 alternate emptying of certain cells, and the filling or turgescence of others, 

 and vice versa the transfer of fluids in this case from cell to cell taking- 

 place in accordance with osmosis. Certain of the tissues are erectile, left 

 to themselves they have a tendency to extend in all directions ; certain 

 other of the tissues are by comparison passive, and act as a check on the 

 erectile tissues. Speaking generally, the spongy cellular portions, such 

 as the pith and the cellular portions of the bark, are erectile ; while the 

 skin or epidermis and the fibrous stringy tissues (wood) are passive. To 

 varying relative conditions of these parts the movements of plants may, 

 in great measure, be attributed. For instance, the divergence which 

 occurs when a Dandelion or other herbaceous stem is cut down, depends 

 on the sudden disturbance of the balance heretofore existing between the 

 erectile central portions and the passive outer portions of the skin : the 

 latter gain the advantage, and curvature results, owing to the adhesion 

 of the cells to the rind ; but if, as in Dr. Bird's experiments, the advantage 

 of the central tissues is restored by the imbibition of fluids, the cut sur- 

 faces then lose their curvature and become parallel to each other. And 

 so, in regard to our second illustration, the contraction that takes place 

 when a ring of bark is removed from the subjacent cells affords another 

 illustration of the difference in tension of the different elements of the 

 stem. 



The tension which occurs in plants is always liable to fluctuations in 

 degree, and may either be permanent or transitory. Permanent tension is 

 that which occurs as a result of unequal growth, while transitory ten- 

 sion is due to variations in the quantity of water absorbed, as in the ex- 

 periments just alluded to. There is always a tendency to maintain an 

 equilibrium ; the tensive forces are, in theory, at least, equally and sym- 

 metrically distributed, but this equal distribution is continually being in- 

 terfered with by variations in the vigour of growth and the amount of 

 light, heat, and' moisture to which plants are subjected. 



It has been long known that if a growing shoot be shaken several times 

 in succession, a curve will ultimately be formed in it, the centre of which 

 curve corresponds with the place where the tension is at its maximum. 

 The irritation produced by the continued shaking stretches the passive 

 tissues (epidermis) and diminishes their elasticity and consequent power 

 of resistance to the erectile tissues within, which are. in consequence, 

 enabled to grow the more rapidly. If this stretching of the rind be uni- 

 form on all sides of the shoot, a simple lengthening will take place ; but 

 of, as is usually the case, the stretching be more on one side than the other, 

 then a curve is produced. 



Periodic Variations in Tension. The degree of tension varies, as has been 

 said, at different times of the day and in different parts of plants. Under 

 ordinary circumstances, according to Kraus, the tension diminishes in 



