TRANSACTIONS OF SECTION K. 723 



high trees only grow where there is a possibility of the access of water-vapour 

 to their roots. When either drought or low temperature or an excess of liquid 

 water decreases the vapour supply beyond a certain limit, trees disappear, while 

 the plants occupying these habitats show marked xeromorphy. 



The theory assumes that the root is divided into two regions : (a) the root 

 hair zone, specialised for salt absorption, which is permeable to liquid water or 

 ' hydropermeable ' ; and {b) the growing region of the root-tip, specialised for 

 water absorption, which is impermeable to liquids but permeable to water- 

 vapour. Both assumptions are justified by experiment. 



The membrane of the root-tip has been called ' aeropermeable ' because it is 

 permeable to gases but not liquids, and its activities are responsible for the 

 pumping efficiency of the root. A membrane with these properties has not been 

 artificially prepared. But it is possible to deduce its qualities mathematically 

 by comparison with the known semipermeable membranes. Its most important 

 function is to demonstrate the pressure of the substance it encloses, i.e., liquid 

 ■water. The total internal pressure of liquid water has been calculated by Van 

 der Waals and Stefan, and reaches magnitudes of 20,000 atmospheres at low 

 temperatures. Because this pressure is normally nullified by cohesion it can 

 only become available when, by a valvular device of molecular dimensions, the 

 liquid is allowed to pass in one direction and not the other. This device is 

 afforded by the aeropermeable membrane. Through it liquid pressure may be 

 generated by a chemical reaction {e.g., respiration) which produces water at 

 the expense of a gas, or by a physical change of state producing liquid from 

 the condensation of vapour. 



The latter mechanism, according to this theory, is responsible for root- 

 pressure. The condensation of vapour is effected by the lowering of vapour- 

 pressure caused by the osmotic substances in the cell solution. This is suffi- 

 cient to counteract the increase of vapour-pressure caused by internal hydro- 

 static pressure. 



The condensation involves a direct conversion of energy and 90 per cent, of 

 the latent heat evolved in the process (580 cals. per gramme_ at 20°) may, 

 theoretically, be converted into liquid pressure. This energy is sufficient to 

 raise two hundred times the weight of the condensed water to 100 metres, allow- 

 ing that the frictional resistance to flow in the xylem is five times the hydro- 

 static head. The ultimate source of energy is the combined action of capillarity 

 imbibition and solar heat on soil particles of different sizes, which produces a 

 slight supersaturation in the interstices of the soil. 



The second function of the root — the absorption of nutrient salts — is sup- 

 posed to be, effected by the alternate extrusion and absorption of an acidic 

 solvent through the root hairs. The conflicting evidence on the presence of this 

 acid excretion is reconciled by the recognition of the aeropermeable membrane 

 in the root-tip. 



Further, it is suggested that the movement of water in plants is a circu- 

 lation, in which the whole upward current travels in the xylem and the whole 

 downward current in the phloem, while the medullary rays serve to maintain 

 the continuity at every level. 



3. The Effect of Temperature on the Permeabilitij of Protoplasm to 

 Water. By Dr. E. Maeion Delf. 



By the use of the optical lever and an apparatus designed by Dr. _F. F. 

 Blackman it has been possible to observe minutely the gradual contraction of 

 a plant tissue undergoing plasmolysis, and thus to determine the rate at which 

 water passes out through the protoplasm by exosmosis. By making these 

 observations at different temperatures, the effect of temperature on the perme- 

 ability of protoplasm has been investigated. 



The behaviour of plant tissue during plasmolysis depends considerably upon 

 the strength of solution emploved. With dilute solutions of cane sugar and 

 dandelion scapes, the course of plasmolysis at any temperature is represented 

 approximately by a logarithmic curve. Curves of this type were obtained at 

 different temperatures ranging from 8° to 43" C., and the Tate of passage of 



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