DEPARTMENT OF BOTANICAL RESEARCH. 83 



The Direct Effect of Rainfall on Tropical Hygrophilous Vegetation, 



by Forrest Shreve. 



A report has been prepared for publication of work carried out in 

 the montane rain-forest of Jamaica, directed to an investigation of 

 the possible physiological effects of prolonged wetness on the foliage 

 of the thin-leaved plants of the rain-forest. The wetting of leaves was 

 found to lessen the amount of their intake of water from the stems, 

 and apparently also to lessen their transpiration. The lessening of 

 intake was found not to be due to the cooling of the leaf, which is 

 slight under rain-forest conditions of humidity, but to be due chiefly 

 to the stoppage of the transpiration which normally takes place through 

 the stomata-free upper surface of the leaves, and also partly to the 

 intake of rain-water by the leaf-surface itself. The wetness of foliage 

 serves, accordingly, to lessen the entry into the leaf of root-absorbed 

 water, with its dissolved content of inorganic salts, and to increase the 

 entry of rain-water, which is practically salt-free. The influence of 

 such impoverishment of the intake of salts can not fail to be of great 

 importance in connection with several of the fundamental processes 

 that take place within the leaf, and it has been correlated with the 

 slow rates of growth that have been shown to take place in the rain-forest. 



The investigation of the dripping-point, or drip-tip, has shown that it 

 is without functional value as a means of hastening the drying of foliage 

 in the rain-forest. The growth of small liverworts, lichens, and mosses 

 on the leaves of plants — a common and deleterious phenomenon — has 

 been shown to be a function of the humidity conditions surrounding 

 the plant rather than a function of the possession or absence of dripping- 

 points. Hydathodes, or water-exuding pores, are of relatively infre- 

 quent occurrence in the Jamaican rain-forest, although the physical 

 conditions are such as would give them the functional utility they have 

 been supposed to possess. Cases of the complete injection of water 

 into the parenchyma of leaves were observed in the rain-forest after 

 exceptionally heavy rain, and the plants with hydathodes were found 

 to be injected, although the prevention of this injurious condition has 

 been alleged to be the chief function of these structures. 



The Non-absorhing Atmometer, by B. E. Livingston and J. W. Shive. 



The porous-cup atmometer (the only instrument so far available 

 for adequate quantitative study of the aerial environment of organisms, 

 so far as the water relation is concerned) has continued to attract 

 attention. Mr. J. W. Shive, fellow in plant physiology in the Johns 

 Hopkins University, has greatly improved the non-absorbing mount- 

 ing for this instrument. It will be remehabered that this mounting 

 involves, essentially, an arrangement by which water moves readily 

 from reservoir to evaporating surface, but not in the opposite direction. 



