How Plants Draw in Various Materials 187 



their osmotic action in withdrawing water from the walls of the 

 stomach, whose dryness, from whatsoever cause, gives the thirst 

 sensation. And there are doubtless other familiar osmotic 

 phenomena which will occur to the ingenious reader, who can 

 now have the pleasure of undertaking their explanation upon an 

 osmotic basis. 



To complete our discussion of water absorption by plants, we 

 must consider the case of dry tissues like wood. Dry wood, as 

 everyone knows, absorbs water eagerly and powerfully, swelling 

 considerably in the action. The conditions for osmosis are ab- 

 sent, and all evidence goes to show that the absorption is due to 

 imbibition into the solid cell-walls. This helps to explain a 

 common phenomenon in connection with wood, — its warping. 

 When water is placed on one side of a dry board, the board warps 

 away from the wet side, often with power enough to tear it from 

 firmly-fixed fastenings; but if the supply of water be continued, 

 the board later flattens out, and a measurement will show that the 

 saturated board is considerably larger than when dry, precisely 

 as a membrane is. Evidently, the water forcibly absorbed by 

 imbibition upon one side forces apart the micellae and swells 

 the wood on that side before it has time to reach the other, al- 

 though, after the lapse of enough time, it penetrates to the other 

 side, swells that, and thus straightens the board, as represented 

 diagrammatically by a combination of the figures 59 and 64. It 

 will here occur to the reader, incidentall}^ that boards often warp 

 without access to water, and simply from the one-sided action of 

 heat. The principle, nevertheless, is the same; even the dryest 

 boards contain some water, the drying of which from one side 

 allows the water remaining in the other to w^arp the board in the 

 usual manner. Furthermore, the reader may recall that a board 

 will warp crosswise but never lengthwise, which fact is correlated, 

 obviously, with another well-known fact about wood, — a fact 

 of very great importance in building and carpentry, — viz., that 

 wood does not lengthen or shrink lengthwise as it does so freely 



