DEPARTMENT OF BOTANICAL RESEARCH. 75 



the miiiiinum average diameter to the maximum being as 1 to 1.5 

 in ferns and as 1 to 13.3 in flowering plants. The weakness of the 

 movements, together with the high cuticular water-loss, serves to 

 give the stomata a very negligible role as regulators of transpiration- 

 rate, particularly during the daylight hours. Indeed, a few instances 

 have been recorded in which a fall in transpiration-rate, during a 

 2-hour interval, was accompanied by an increase in stomatal area. 

 No direct evidence has been secured to show that transpiration-rate 

 itself controls stomatal movement. 



The securing of relative transpiration data has made possible a 

 comparison of the transpiration beha\aor and the transpirational 

 water-losses of different individuals of the same species, rain-forest 

 plants of different habitats, and also of rain-forest and desert plants. 

 Individuals of the same species exhibit, in general, the same daily 

 behavior, but the amounts of their water-loss per unit area are often 

 unlike, and are so without discoverable differences in the gross 

 anatomy or environmental conditions of the individuals. This cir- 

 cumstance has made it necessary, for some purposes, to calibrate 

 plants by preliminary measurements of their transpiration, and to 

 compare the water-losses of different species by averaging the results 

 from several individuals. The different species, characteristic re- 

 spectively of the very moist ravines, of the rather moist slopes, and 

 of the wind-swept and relatively dry ridges of the rain-forest, when 

 investigated under the same conditions, showed transpiration 

 amounts in the proportion respectively of 3.5, 1.6, and 1. The 

 evaporation conditions of these habitats differ, as measured during 

 several months, by amounts which are closely equal to a reversal 

 of the above proportions. This is to say, the actual amounts of 

 water-loss in the plants, when growing in their natural habitats, is 

 approximately the same per unit area. 



A comparison has been instituted between the relative transpira- 

 tion data for the rain-forest plants and those secured at Tucson, by 

 Livingston for several desert ephemerals and for Parkinsonia in dif- 

 ferent vegetative conditions by Mrs. Edith B. Shreve. The most 

 nearly xerophytic of the rain-forest plants exhibit about the same 

 maximum relative transpiration-rates as do the most hygrophytic 

 of the desert herbaceous species. The relative rates for herbaceous 

 plants of the rain-forest, as determined in the shade, are about half 

 of the rates for the desert ephemerals, as determined in the sun, and 

 there is some evidence that this difference is due to the different light 

 conditions in which the two sets of experiments were performed. 

 The rates for Parkinsonia, determined in the sun, are of about the 

 same general order of magnitude as the shade rates for the Jamaican 

 herbaceous species. A general review of the data under comparison 

 indicates that, in spite of minor differences, there is a greater uni- 



