INTRODUCTION. 7 



begins. Chlorophyll is abundant in all parts of the bark, except at the 

 short base of the main trunk, so that the death of any part of the tree is 

 easily detected by a change in color of the bark from bright green to brown. 

 Certain dead twigs were marked, and in the following year the dead region 

 was found to have extended farther down the stem, but no case was observed 

 where dying had begun below the tip. After a limb is dead it is of course 

 impossible to tell the stages by which it died, but the number of dead twigs 

 and short dead branches existing on live branches makes it seem probable 

 that death begins at the tips and progresses down the stem gradually. The 

 death of an entire limb seems in no way to affect the health of the plant as 

 a whole. 



As is the case with most of the Leguminosae, the plant possesses leaf- 

 movements. During darkness the leaflets are " closed " that is, they stand 

 with their dorsal surfaces in parallel planes about 0.1 cm. apart and with 

 the coming of daylight the leaflets separate, making various angles with the 

 rachis, the "wide-open" position being when the leaflets lie in the same 

 plane on opposite sides of the rachis. During cool or moist seasons the 

 leaflets remain "open" or partly open during the day, but during the dry 

 seasons of spring and fall they close within half an hour after sunrise and 

 remain in this position all day, sometimes opening for about half an hour 

 near sunset. Leaflets from potted plants that have been well watered open 

 wider as a rule in the morning than do those from adult trees and are slower 

 in closing. If, however, the potted plants are not given water for several 

 days the movements are more like those of the leaflets of adult trees. 



The size of leaflets varies, both in area and thickness, with the age of the 

 plant and with the soil and atmospheric conditions existing during the period 

 of their growth. On seedlings from one to eight years old the area of one 

 side of a leaflet averages 0.04 sq. cm. ; on an adult tree the average is 0.02 

 sq. cm. ; and on hot-house-grown seedlings a year old it is 0.06 sq. cm. The 

 leaflets which come from very small branches emanating from the main 

 trunk near the base are frequently about the same size as those on young 

 plants. The diameters are approximately in inverse ratio to the area. 

 Figure 1 shows the diameters and arrangement of tissue in leaves of dif- 

 ferent types. The different drawings are on the same scale and were made 

 with a camera lucida. 



The following differences of structure may be concerned with the trans- 

 piration studies which follow. The section of the leaflet from an adult tree 

 (A, fig. l) shows large epidermal cells with heavily thickened walls, stomata 

 about midway down the first row of cells, very few intercellular spaces, palisade 

 cells and stomata on both sides of the leaf. The section from a plant grown 

 all its life in the green-house (C,fig. 1) shows no thickening of the epidermal 

 cells, many large intercellular spaces, palisade cells on the dorsal side only, and 

 stomata on both sides. The section from the leaf grown in the green-house for 

 a year and then placed out of doors for six weeks (5, fig. l) shows a structure in 

 several ways intermediate between the other two. The epidermal cells are 

 thickened nearly as much as in the section from the adult tree; palisade cells 



