iQ2o] CURRENT LITERATURE 93 



field is by McLean, 9 who worked in the rich forests on the slopes of the hills 

 near Rio de Janeiro, Brazil. This is a region of high average humidity, due 

 to a rainfall of 111.2cm., the heaviest downfall being during the warmer 

 months, and to a very considerable amount of cloudiness upon days with no 

 rainfall. Considerable climatological data are presented, and a graph of 

 climatic favorability is devised by combining the four factors of temperature, 

 rainfall, relative humidity, and sunshine. The curve of this graph seems to 

 show that the year may be divided into a more and a less favorable period, the 

 latter extending from June to December. 



Atmospheric humidity is shown to be high, even outside the forest cover. 

 Graphs are presented showing the relative range of humidity and temperature 

 at various levels of the vegetation. The latter records prove that a dense 

 layer of shrubs divides the forest into two strata, the lower possessing cooler 

 and more humid conditions than the lighter and better ventilated regions 

 above. The author believes that this lower stratum is the less favorable to 

 vegetation, and to it his experimental work is confined. 



Transpiration measurements by means of potometers give the water loss 

 by leaves in the lower stratum of the forest always less than o . 4 of the evapo- 

 ration from a free water surface exposed alongside the foliage. Experiments 

 within the laboratory with similar temperature and humidity, but with higher 

 illumination, are shown to give similar results. Many of the shade leaves 

 possess an amount of cutinization that reduces cuticular transpiration to a 

 very slight amount. Structural studies show the intercellular spaces of sun 

 and shade leaves to be relatively 16.3 and 24.8 per cent, and these amounts 

 correspond very closely to those found in Europe. The size and amount of 

 stomata seem to be rather decidedly smaller than that found in typical meso- 

 phytes of temperate lands. The vascular strands of the shade leaves are much 

 smaller in cross-structure than those of sun leaves. These data, and the fact 

 that the author believes the power of root absorption to be low, make it 

 probable that, even in the protected region of the lower interior of the forest, 

 transpiration may for short periods decidedly surpass the low capacity of the 

 plants to supply water. This is supposed to account for cutinization, semi- 

 succulence, and other xeromorphic tendencies and features of the tropical forests. 



Under such conditions of reduced transpiration, however, there is no short- 

 age of mineral matter, but on the contrary the leaves from shaded and pro- 

 tected habitats show relatively a richer content than do those sun forms with 

 a much higher transpiration rate. This would prove that here at least the 

 absorption of mineral salts is quite independent of any transpiration current. 



A study of the foliage proves the predominance of the lanceolate leaf form 

 and a remarkable prevalence of nyctitropic folding, which, however, does not 



9 McLean, R. C, Studies in the ecology of tropical rain forests; with special 

 reference to Brazil. I. Humidity. Jour. Ecol. 7:5-54. Pi- i-fig$- 21. 1919. 



