DEPARTMENT OE BOTANICAL RESEARCH. 6l 



a union of tissues is effected although the junction may be indirect. The 

 parasitic relation under discussion is such that the dependent partner of the 

 union receives liquid material through its own proper absorbing organs. A 

 comprehensive series of chemical tests is being made to ascertain the acidity 

 and salt-content of the plants used as host and dependent in these experi- 

 ments. The osmotic pressure is probably the most important limiting factor 

 in all dependent nutritive arrangements. 



The recent discovery of parasitism by green plants ordinarily autophytic 

 suggests that this relation may originate fortuitously and the experimental 

 results obtained promise some definite information as to the conditions 

 favorable to such a change in seed-plants, which should also be considered in 

 other parasitic arrangements. 



Grozvth and Alterations in Form and Volume of Succulents (by Mrs. E. S. 

 Spalding) : 

 The series of measurements by which the principal changes in form and 

 volume of some of the larger cacti have been detected have now been 

 carried through a fifth year. The data obtained show that the water-balance 

 of the succulents of the Arizona deserts increases to a maximum during 

 the two rainy seasons and decreases during the two intervening dry periods, 

 being accompanied by changes in form and size which may be readily cali- 

 brated. In addition to actual shrinkage and swelling, the external ridges of 

 columnar or cylindrical cacti exhibit specialized movements. These, as well 

 as flower-formation and growth, are influenced by local exposure, so that the 

 northern and southern sides of a plant display marked differences. Some of 

 the changes are not readily assignable to known causes. The complete results 

 will be presented in a paper now in course of preparation. 



Relation of Evaporating Capacity of the Air to Plant Distribution (by Dr. F. 

 Shreve) : 

 A secure basis for studies in plant distribution is to be found only in the 

 results of analyses of habitat conditions, and Dr. Shreve has cooperated 

 extensively with Dr. Livingston in the reduction of atmometric data from 

 which a vegetational map of North America has been prepared. 



Activities of Plants of Tropical Rain-Forests (by Dr. Forrest Shreve) : 



Dr. Shreve has spent the latter half of the year at the Tropical Laboratory 

 of the New York Botanical Garden, at Cinchona, Jamaica, where a continu- 

 ation of his studies upon the growth, transpiration, and photosynthesis of 

 certain species has been carried out. Special attention has been paid to the 

 influence of high relative humidity and cloudiness upon the collective activi- 

 ties of tropical forms to obtain data for contrast with observations made in 

 the desert. The rate of transpiration is found to follow closely the rate of 

 evaporation, stomatal movement being always very slight and relatively un- 

 important as a regulative factor. 



