68 Journal of the Mitchell Society [August 



critical period for the plant with the result that it is then that the 

 maximum sap densities of the growing season occur. 



Determinations made in the middle of the Avinter, on the other 

 hand, showed the conifers to have low concentrations, while the closely 

 associated evergreen shrubs exhibited high concentrations. Further- 

 more, the sap densities of the shrubs increased with an increase in 

 elevation — a complete reversal of the results obtained during the 

 growing season. Inasmuch as the winter densities were so completely 

 at variance with those of the growing season a special micro-chemical 

 study was made to determine the underlying causes. The writer was 

 led to conclude that with the advent of cold weather in the autumn 

 and early winter, a large part of the starch in the conifers is con- 

 verted into oil or fatty substances which are osmotically inactive and 

 form emulsions having low osmotic concentrations. In the evergreen 

 shrubs, which showed little or no oil present, the starch was evidently 

 converted into soluble sugars, thereby materially increasing the os- 

 motic concentration of the cell sap. 



In summarizing, it may be said that these studies showed that the 

 osmotic concentration of the cell sap of plants may be used as an index 

 of habitat in correlating the great complex of habitat factors with the 

 physiological responses of the plant. The density of the sap of a 

 species, however, is not constant. It may be influenced by any of the 

 environmental conditions affecting transpiratioUj the products of 

 photosynthesis or the supply of available soil moisture. Osmotic pres- 

 sure in plants is more rapidly changed by fluctuations in the moisture 

 conditions of the habitat than by temperature or light. During the 

 growing season the lowest sap concentrations occur in those meso- 

 phytic plant associations which are successionally the most highly 

 developed with reference to an adequate supply of available moisture 

 and in which the complex of conditions is most favorable to plant 

 growth. On the other hand, the highest densities occur on the most 

 adverse sites, either the driest or the most saline. 



In North Carolina, although a maximum altitudinal range of 

 6,700 feet is encountered in passing from the eastern coastal plain 

 swamps and pine lands to the spruce-fir forests on the higher moun- 

 tains of the western end of the state, the extreme concentrations noted 



