How Plants Draw in Various Materials 173 



We return for a moment to our osmoscope, for such is the name 

 of our osmosis-exhibiting instrument. As the liquid ascends in 

 the tube, a brown color appears in the water outside, showing 

 that some of the molasses comes out, though of course in much 

 smaller amount than the water which enters, else the liquid could 

 not rise in the tube. This suggests at once the inquiry, does the 

 sugar in the sap of the root-hair cells also come out into the soil? 

 It does not, as ample evidence attests. And if we seek in parch- 

 ment cup and root hair for a structural difference to explain this 

 difference in osmotic action, we can easily find it; for the hairs 

 possess a complete lining film of living protoplasm to which 

 there is no equivalent in the parchment cup. It is easily shown by 

 experiment that this protoplasm really does stop the passage of 

 sugar while permitting that of water; and this fact explains not 

 only why no sugar passes out of the hairs into the soil, but also 

 the equally striking phenomenon, that none passes out of the 

 cortical cells into the ducts, for in general it is only pure water 

 which ascends through the ducts to the leaves. Protoplasm, 

 however, is not the only membrane of this type (which, because 

 permeable to water but not to dissolved substance is called semi- 

 permeable, in distinction from the ordinary kind which are per- 

 meable to both), for they can be constructed artificially from chem- 

 icals, and even laid down in a uniform film all over the interior 

 face of the parchment cup. In this case our osmoscope becomes a 

 very close physical duplicate of the living root hair, and likewise 

 permits the steady absorption of water without the escape of any 

 of the sugar whatsoever. My students have often constructed 

 such arrangements, with results that were wholly satisfactory. 



If, now, the reader will compare point by point, an osmoscope 

 containing a semipermeable membrane, and the absorbing mech- 

 anism of the living root (which is diagrammatically represented 

 in figure 55), he will agree that they match very closely in physical 

 construction and operation except for one very notable difference, 

 namely, while the liquid which rises in the osmoscope is a mix- 



