July 13, 1917] 



SCIENCE 



29 



us at once measures of the physiological 

 characteristics of these two forms. These 

 quantitative measures furnish a first defi- 

 nite step towards the solution of the prob- 

 lem of their distribution. 



To discuss adequately the many prob- 

 lems presented by a comparison of the sap 

 properties of the vegetations of diverse 

 local habitats or phytogeographical regions 

 wojild carry us far beyond the limits of this 

 address. Before leaving this phase of the 

 subject it is important to point out that in 

 its relationship to plant distribution, sap 

 concentration may have a dynamic as well 

 as a static significance. 



If the differences in the sap properties 

 of the vegetations of various habitats be in 

 part due to fixed hereditary differences in 

 the species, instead of merely a resultant 

 of local environmental conditions, and if 

 one of the factors determining the capac- 

 ity for survival in a given habitat be the 

 osmotic concentration of the cell sap, it is 

 clear that sap properties may be a factor in 

 the migration of species. 



The factor of osmotic concentration 

 would be active in two ways. First, in the 

 detennination of migration from warmer 

 into colder regions, by virtue of capacity 

 for frost resistance. Second, in migration 

 from mesophytic into desert regions. 



The problem of the relationship of sap 

 concentration to frost resistance need not 

 delay us long. The freezing-point of plant 

 tissues has been the subject of scores of 

 investigations, most of which have been of 

 a purely physiological or of an economic 

 nature. 



The studies of Ohlweiler, Chandler and 

 others render it highly probable that the 

 osmotic concentration of the tissue fluids 

 is one of the factors involved in the capac- 

 ity for frost resistance. Such determina- 

 tions as Mr. Popenoe and I have been able 

 to make on the sap of the varieties of 



avocado (Persea americana) which have 

 been introduced into the United States, 

 indicate that the Mexican and Guatamalan 

 types, which have been found by practical 

 horticulturists to surpass the so-called 

 West Indian type in capacity for frost re- 

 sistance, have a slightly higher osmotic 

 concentration of their cell sap. 



Concerning the role of osmotic concen- 

 tration in the survival of plants intro- 

 duced into xerophytic regions we have as 

 yet practically no information. 



It is perhaps evident that the factors 

 which limit the artificial introduction of 

 species would also be active in determining 

 the survival of species introduced into any 

 region by hurricanes, ocean currents or by 

 any other natural causes. Upon some of 

 these questions I hope to be able to furnish 

 more satisfactory information on a later 

 occasion. 



"We must now turn to a discussion of cer- 

 tain of the ecologically more interesting 

 groups of plants. Among these may be 

 mentioned the succulents, the epiphytes 

 and parasitic plants. 



The studies of succulent physiology 

 which have been carried out in recent 

 years, and especially at the Desert Labo- 

 ratory by MacDougal, Spoehr, Richards, 

 Mrs. Shreve and others, have been far too 

 detailed to make possible any adequate 

 discussion of succulency at this time. It 

 is interesting to note in passing that the 

 succulents are characterized by two quite 

 different types of sap. On the one hand 

 are the desert species with generally low 

 osmotic concentration, on the other the 

 halophytes with high osmotic concentra- 

 tion. The physiological interpretation of 

 this condition presents a most interesting 

 problem for future research. 



Among the most characteristic, and 

 ecologically most fascinating features of 

 tropical regions is the burden of epiphytes 



