52 CARNEGIE INSTITUTION OF WASHINGTON. 



The capacity of shoots of cacti for water increases up to maturity 

 at an age of a year or more, with the dry weight, then decreases. 

 Disks cut from the joints of young platyopuntias increase in thickness 

 about 23 per cent in water and in alkaU, but only 16.4 per cent in acid, 

 while samples from plants a year old swelled 40 per cent in water, 52 

 per cent in alkaU, and 36.6 per cent in acid. 



A search for a mixture of colloids that would exhibit imbibition similar 

 to those of the plant disclosed that gelatine with a smaller quantity of 

 agar makes such a mass. Gelatine swells most in acid, and agar in 

 distilled water. The mixture shows greatest swelUng in alkali, and 

 simulates the plant as to general composition and beha\'ior. 



The rate of transpiration, movements, etc., is influenced by the 

 capacity of the plant for imbibition. The water-holding capacity of 

 tissues of cacti is less at night than in the daytime, although the sto- 

 mata are open at night and closed during the day. 



The relative parts which might be played by imbibition and turgor 

 in the growth of pollen-tubes in cane sugar and acids and alkalies 

 were studied. The colloids in these cells are of a kind which swells 

 more in acid than in alkali in very low concentrations. 



The ratio of precipitation to evaporation is found to be the factor 

 which limits the areas of forest, grasslands, and desert, and a new 

 subdivision of the vegetational areas of the United States has been 

 worked out, and with this a new method of determination of the 

 domination of different growth-fonns has been devised and put into use. 



Flattened bodies of plants like those of the platyopuntias in a merid- 

 ional position receive more heat in the course of a day at Tucson than 

 those in an east-and-west position, and attain temperatures of 53° C. 

 (124° F.) or more. Transpiration, dry weight, etc., correspond to the 

 exposure. 



The roots of different plants show varied relations to oxygen, which 

 is correlated with soil-penetration and habit. Similar differences as 

 to temperature at which growth proceeds have been measured. The 

 forms taken by root-systems of plants may be greatly affected by 

 change in environmental conditions. 



The roots of the swamp willow {Salix sp.) grow for extended periods 

 in atmospheres devoid of oxygen. 



For study in connection with the physical characters of their habi- 

 tats, 300 determinations have been made of the concentration of the 

 saps of plants in mountain and desert habitats. The sap of parasites 

 is generally of a higher concentration than that of the host, though 

 not always or necessarily so, as suggested by MacDougal in 1910. 



A map of the vegetational areas of the United States, based on 

 behavior and anatomy of growth-forms, has been brought to the publi- 

 cation stage. 



Beetles (Leptinotarsse) introduced at Tucson show gradual morpho- 

 logical changes which do not seem to be reversible, and which react as 



