CIRCULATION AND FORMATION OF WOOD IN PLANTS. 593 



which, in the spring, while yet there is no considerable evaporation 

 to aid it, goes on with a power which capillarity does not explain. 

 The co-operation of the same two agencies is assignable for this result 

 also.* The circumstance that vessels and ducts here contain sap and 

 there contain air, and at the same place contain at different seasons 

 now air and now sap is a fact calling for explanation. An explana- 

 tion is furnished by these mechanical actions which involve the- en- 

 trance or expulsion of air according to the supply of liquid. That 

 vessels and ducts which were originally active sap-carriers go com- 

 pletely out of use, and have their function discharged by other 

 vessels or ducts, is an anomaly that has to be solved. Again, we 

 are supplied with a solution : these deserted vessels and ducts are 

 those which, by the formation of dense tissue outside of them, 

 become so circumstanced that they cannot be compressed as they 

 originally were. A channel has to be found for the downward 

 current of sap, which, on any other hypothesis than the foregoing, 

 must be a channel separate from that taken by the upward 

 current ; and yet no good evidence of a separate channel has been 

 pointed out. Here, however, the difficulty disappears, since one 

 channel suffices for the current alternating upwards and downwards 

 according to the conditions. Moreover there has to be found a 

 force producing or facilitating the downward current, capable even 

 of drawing sap out of drooping branches ; and no such force is 

 forthcoming. The hypothesis set forth dispenses with this necessity ; 

 under the recurring change of conditions, the same distention and 

 oscillation which before raised the sap to the places of consumption, 

 now bring it down to the places of consumption. A physical 

 process has to be pointed out by which the material that forms 

 dense tissue is deposited at the places where it is wanted, rather 

 than at other places. This physical process the hypothesis in- 

 dicates. It is requisite to find an explanation of the fact that, 

 when plants ordinarily swayed about by the wind are grown indoors, 

 the formation of wood is so much diminished that they become ab- 

 normally slender. Of this an explanation is supplied. Yet a further 



* It seems probable, however, that osmotic distention is here, especially, 

 the more important of the two factors. The rising of the sap in spring may 

 indirectly result, like the sprouting of the seed, from the transformation of 

 starch into sugar. During germination, this change of an oxy-hydro-carbon 

 from an insoluble into a soluble form, leads to rapid endosmose ; con- 

 sequently to great distention of the seed ; and therefore to a force which 

 thrusts the contained liquids into the plumule and radicle, and gives them 

 power to displace the soil in their way : it sets up an active internal move- 

 ment when neither evaporation nor the change which light produces can be 

 operative. And similarly, if, in the spring, the starch stored up in the roots 

 of a tree passes into the form of sugar, the unusual osmotic absorption that 

 arises will cause an unusual distention — a distention which, being resisted by 

 the tough bark of the roots and stem, will result in a powerful upward thrust 

 of the contained liquid. 

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