106 Hydration and Growth. 



The swelling of dried sections is clearly under conditions exclusive 

 of the action of a semi-permeable membrane, except in so far as 

 the cell-walls may show such properties. If desiccation resulted in 

 simple loss of water such as that which ensues in an unsalted plate 

 of biocoUoids, the action of living and dried material might be expected 

 to be identical. The presence of salts and acids, however, causes some 

 irreversible changes, and the relative swelUng of dried sections in various 

 solutions is different from that of a series obtained from the use of liv- 

 ing cell-masses. That the cell does act as an osmotic machine is estab- 

 lished beyond all question. That it is an enormously complex system 

 of osmotic sacs is well estabUshed. That the differential action of the 

 specialized layers formed at all phase boundaries can be made to 

 account for the entire relations of the protoplast is more than doubtful. 

 While all hydration in the broadest sense, including osmotic action, 

 ultimately depends upon molecular affinity, it is evident that the con- 

 ception of the semi-permeable sac does not offer a suite of possibilities 

 which may account for the range of action of the protoplast. 



A second proposal is that which has been most clearly outhned by 

 Dr. E. E. Free, based upon the general acceptance of specialization of 

 colloidal conditions in the external layer of the protoplast, but explains 

 its action upon changes in the relations of the colloidal phases in it.^ 



Shrinkage of such a cell would follow a change in the dispersion or 

 phase relations of the external layer, and might be accompanied by the 

 solvation or passage out of the cell of substances which would reduce 

 its water-holding capacity still further. The explanation based on 

 osmosis might account for the escape of the free liquid of the vacuoles, 

 or from syneretic cavities, but some mechanism such as that suggested 

 by Free is necessary to account for the lessening of the amount of 

 water held by the cell colloids. 



Some further evidence on the matter of extraction of substances 

 from colloidal masses and the action of contained substances on desic- 

 cating colloids remains to be considered. The facts which seem to 

 warrant the prolongation of the discussion were obtained by a com- 

 parison of the results of swelhng of Uving and dried sections, with 

 determinations of their acidity, measurements of substances extracted, 

 and measurements of repeated swelUngs of the same material. 



These treatments as appUed to median slices of maturing joints of 

 Opuntia discata grown at Carmel gave measurements at 18° C. as 

 shown in table 83. 



The second swelling of dried sections which had been once swelled 

 (extracted) indicate that some material of high hydration capacity had 

 been dissolved out, as the sections which had been simply dried swelled 

 25 times as much in potassium nitrate. Material which was dried 

 directly without extraction did not undergo any lessened hydration 



'Free, E. E. A colloidal hypothesis of protoplasmic permeability. Plant World, 21:141. 1918. 



