DISTRIBUTION OF WATER IN THE WOOD. 341 



Consequently the volume of the cell-cavities is calculated as — 

 1 6*56 cubic centimetres containing air. 

 + 46*23 „ containing water. 



= 62 '79 cubic centimetres of cavity altogether. 

 The volume of the saturated cell-walls as — 



2 4 "8 1 cubic centimetres dry mass of waif, 

 -h i2"4 „ water of imbibition. 



= 37'2i cubic centimetres saturated wood walls. 



The space occupied by the saturated walls, therefore, in this case stands in the 

 proportion to the space occupied by the water and air, as i : i"68; or, the space oc- 

 cupied by the saturated walls is greater than a third of the total volume of the wood. 



It is to be noticed that in this particular case the piece of wood examined 

 contained much water. Had less water been present the calculated space containing 

 air would have turned out to be greater, since we may assume that so long as water 

 is still present in the cavity of the cells, ''the cell- walls themselves are completely 

 saturated with it '. 



The calculation given depends in part on the assumption that cell-walls, taken 

 as dry, only imbibe water to the extent of the half of their volume. 1 was led to this 

 assumption by the following considerations and results : if a thin cross slice of fresh 

 wood is suspended in dry air, there is generally formed during the drying of the wood 

 a radial fissure, which forces its way from the exterior to the centre. Hereupon the 

 cross slice is dried at 100° C, and the weight of the wood cell-walls determined; from 

 which its volume is reckoned by means of the specific gravity. The dry slice of wood 

 is now again suspended in a space saturated with aqueous vapour, where it gradually 

 condenses so much water, and at the same time becomes so distended by swelling, 

 that the fissure produced during the drying again closes up, and this so completely that 

 it is finally no longer to be recognised at all. In this condition the cell-walls must 

 necessarily be completely saturated with water ; and there is no fear of water being con- 

 tained in the cell-cavities also. Weighing once more, gives the result that the water 

 thus absorbed until the cell-walls are saturated constitutes about half their volume. 



This last discovered fact is now, however, of particular interest, since it shows 

 that the wood cell-walls have a strikingly small power of swelling, compared with 

 other cell membranes which are capable of swelling, and especially with those which 

 become mucilaginous in water, and take up enormous quantities of that liquid. 



Here, then, we are face to face with the proper and specific physiological signi- 

 ficance of the wood cell-walls. This consists in that they absorb relatively but little 

 water, but that this small quantity of water of imbibition is strikingly mobile in them. 



According to all that has been said hitherto, the ascending current of water in 

 transpiration (in the wood generally) thus moves in the substance of the cell-walls 



' The knowledge of the specific gravity of the wood cell-walls gives us also the power of 

 calculating the extent of surface of all the cell- walls in a given piece of wood. For a piece of fresh 

 Fir-wood in winter I found, for example, that 100 c.cm. contained 25 c.cm. of solid wall. Since now 

 the thickness of a saturated wall may be assumed to average about 0.0025 mui., by dividing the 

 volume of wall named by this thickness it results that the superficial extent of the walls in a 

 piece of Fir wood i m. long and I sq. cm. in diameter = 10 sq. m. 



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