28 



SCIENCE 



[N. S. Vol. XLVI. No. 1176 



How great the differences in the sap prop- 

 erties of the vegetations of various regions 

 may be is illustrated by the accompanying 

 table in which the average concentration in 

 atmospheres for Long Island, Arizona and 

 Jamaican regions studied by my associates 

 and myself and for a series of determina- 

 tions made for a quite different purpose by 

 Ohlweiler at the Missouri Botanical Garden 

 are laid side by side. 



Had it been possible to table the data of 

 each of these regions, and that for others 

 which are now available, according to local 

 habitats it would have been seen that in 

 any region the local habitats may be meas- 

 urably differentiated with respect to the 

 sap properties of their vegetation. Tabu- 

 lation by local habitats would also have 

 brought out clearly the fact that herbaceous 

 and ligneous plants differ in the osmotic 

 properties of their tissue fluids. 



The averages in the table are not pre- 

 sented as complete descriptions of the sap 

 of the plants of these regions, but merely as 

 the simplest available means of summariz- 

 ing their characteristics and emphasizing 

 to the phytogeographer the fact that the 

 vegetations are differentiated in the prop- 

 erties of their sap as well as in their taxo- 

 nomic composition and ecological structure. 



The explanation of such differences in 

 vegetations as have just been demonstrated 

 is by no means simple. 



The most direct and obvious relationship 

 of the properties of the sap of the organism 

 to its environment is to be seen in halo- 



phytes. The leaves of many of these are 

 salty to the taste. It is quite apparent that 

 they must have a relatively high concentra- 

 tion due to the absorption of salts from the 

 substratum. It is, however, a grave error 

 to assume, as some botanists seem to have 

 done, that the whole problem of sap con- 

 centration is one of the absorption of elec- 

 trolytes from the soil — to assume in fact, 

 that the plant organism stands in the rela- 

 tion of a sponge to the solution around it. 

 Zoologists, who have devoted much atten- 

 tion to the relationship between the con- 

 centration of the blood of the marine organ- 

 isms and that of the water in which they 

 live, have long recognized that the osmotic 

 concentrations of the two fluids may be 

 identical but that the solutes to which these 

 concentrations are due may be very differ- 

 ent indeed. 



Even in the succulent halophytes, the 

 leaves of which are essentially reinforced 

 water bags, there appears to be by no 

 means an identical capacity for adjustment 

 to the concentration of their substratum or 

 for the occupation of available areas. Thus, 

 for example, Sesuvium Portulacastrum and 

 Batis maritima both occur on the highly 

 saline flats of the southern shore of the 

 island of Jamaica. Batis shows a far higher 

 osmotic concentration than Sesuvium, 49.7 

 as compared with 38.3 atmospheres on the 

 average, and is seen in the obviously more 

 saline localities. 



Now differences in the concentration of 

 the soil solution may not be the determin- 

 ing factor in the distribution of these two 

 halophytes. Other factors require far more 

 detailed investigation than any one has 

 been able up to the present time to give 

 them. The point to be emphasized here is 

 that two species of halophytes, not with- 

 out several points of similarity, differ in 

 both sap concentration and in local dis- 

 tribution. The chemical method has given 



