556 SCIENCE PROGRESS 



centration and Electrical Conductivity of the Tissue Fluids of 

 Ligneous and Herbaceous Plants," Journ. Phys. Chem., 25, 

 122-146, 1 921) in the case of plants of the Arizona deserts and 

 of the north shore of Long Island. 



About twelve years ago it was suggested by MacDougal 

 and Cannon that there might be a fundamental relation between 

 the osmotic pressure of the cells of parasites and that of the cells 

 of the host plants. That this is indeed the case has been shown 

 by Harris and Lawrence (" On the Osmotic Pressure of the 

 Tissue Fluids of Jamaican Loranthaceae parasitic on various 

 Hosts," ^mer. Journ. Bot., 3, 438-455, 191 6) for various Loran- 

 thaceae of montane rain-forest, and by Harris and Valentine 

 (" The Specific Electric Conductivity of the Tissue Fluids of 

 Desert Loranthaceae," Proc. Soc. Exper. Biol. Med., 18, 95-97, 

 1920) for desert species of the same family. Their results 

 indicate that the osmotic concentration of the cells of the para- 

 site is in general higher than that of the leaf cells of the host, 

 the difference being of the order of from 5 to 20 per cent, in the 

 cases examined. 



Non-parasitic epiphytes, on the other hand, appear to 

 exhibit unusually low osmotic pressures, to judge from the 

 values obtained by Harris (" On the Osmotic Concentration of 

 the Tissue Fluids of Phanerogamic Epiphytes," Amer. Journ. 

 Bot., 5, 490-506, 191 8). In the epiphytic species examined by 

 him the osmotic pressure of the expressed sap appeared to be 

 from 37 to 60 per cent, lower than that of the sap of herbaceous 

 plants and from 28 to 45 per cent, lower than that of ligneous 

 species. 



Bound up with the question discussed in the preceding 

 paragraphs is that of the possible effects of external conditions 

 on the osmotic pressure of plants. The high values found by 

 Harris, Gortner, Hofman, and Valentine for the osmotic pressure 

 of the expressed sap of plants growing in the neighbourhood of 

 the Great Salt Lake, to which reference has been made earlier, 

 is probably connected with the high content of salt in the soil. 

 Differences in the osmotic pressure of the sap of ligneous plants 

 growing in different habitats have been observed by Harris and 

 his co-workers, the average osmotic pressure of the sap of such 

 plants in Jamaican montane rain-forest being 1 1 -44 atmospheres, 

 in Long Island 14-40 atmospheres, and in the deserts of southern 

 Arizona 24-97 atmospheres. 



Some direct observations on the influence of external con- 

 ditions on the osmotic pressure of plant cells have been made 

 by Ursprung and Blum (" Uber den Einfluss der Aussenbeding- 

 ungen auf den osmotischen Wert," Ber. deut. hot. Ges., 34, 

 123-142, 1916). From their results, as well as from those of 

 earlier investigators, it appears clear that a higher water-con- 



