BOOKS AND CURRENT LITERATURE 63 



have, in general, a higher sap concentration than the dwarf shrubs, 

 and the latter a higher concentration than the annuals and root-peren- 

 nials. The only true trees worked on were Olneya, which grows away 

 from water courses, and shows a concentration of 29.7, and Fraxinus, 

 which grows only near streamways and shows a concentration of 16.7 

 to 19.6. The semi-succulent Dasylirion has a concentration of 35.0 

 and the leaf-succulent Agave of 11.1, while the deciduous Fouquieria 

 varies from 10.4 to 14.4. 



The material used by Harris and his collaborators was taken in five 

 habitats differing in physical conditions and in vegetation. These 

 were the floor of a canon, the rocky slopes of a hill, bajada slopes (tilted 

 detrital plains), a sandy arroyo, and a salt spot. The osmotic con- 

 centrations were averaged for all of the plants of each vegetative type 

 that were gathered in each habitat. These are lowest for the arroyo, 

 slightly higher for the canon, still higher for the rocky slopes, again 

 higher for the bajada, and highest for the salt spot. These results 

 indicate that there is an appreciable difference in sap concentration 

 for each type of plant in each of these habitats, which are among the 

 most marked that may be seen at desert elevations in the Tucson 

 region. 



However much the extraction of sap by Harris's method may obscure 

 the conditions under which many of the foliar functions are carried on, 

 it gives nevertheless a means for securing an average expression of the 

 physico-chemical condition of the juices of the plant, which he terms 

 ''both the product and the environment of the activities of the pro- 

 toplast." However much our conception of the importance of osmotic 

 phenomena in plants may be modified by the results in colloidal chemis- 

 try which are now beginning to accumulate, nothing can minimize the 

 importance of the physico-chemical condition of the sap of plants as 

 related to environmental conditions, and particularly to the intake 

 and loss of water. The facts that Harris's results are accordant with 

 our knowledge of the physical conditions of the habitats in which he 

 worked, and with what we know of the seasonal behavior of the plants 

 which he used, are in themselves a confirmation of the physiological 

 soundness of his work, as well as a stimulus to the study of physiological 

 phenomena in connection with field work. The results that have been 

 secured by Harris in his subsequent work in Florida, Jamaica, and 

 Arizona will be awaited with interest. — Forrest Shreve, 



