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of the stomates, and these conclusions were supported by Lloyd (1908) in his 

 observations on Fouquieria splendens. Consequently it is only when the stomatal 

 opening is nearly closed that stomatal regulation can have any effect upon the 

 transpiration rate. This is exemplified in the case of Casnarina lepidophloia. 

 During the day the transpiration curve of this plant closely approximates that 

 for the evaporating power. This is intelligible since the stomates in the furrows 

 are but little protected by the few hairs which are developed. High winds 

 have little difficulty in sweeping along the lines of stomates in the furrows of 

 the vertically placed shoots that bend before them. The lack of agreement of 

 the transpiration with the evaporation curve during the night is ascribed to 

 stomatal control. Livingston (1906) has found similar curves for Euphorbia 

 capitella, a xerophytic Arizonan plant; here the day curves for transpiration 

 approximated that for evaporating power, whilst a lack of agreement occurred 

 at night. 



The plants in the second group — Geijera parviflora and Pholidia scoparia — 

 show their transpiration maxima at the same time as the evaporation maximum, 

 but the curves are much flatter, i.e., they do not respond readily to variations in 

 evaporating power. This reduction is ascribed in both cases to the presence of 

 oily secretions, the vapors of which materially reduce transpiration. Further 

 protection is afforded by the ridge in Geijera parviflora and by the scales in 

 Pholidia scoparia. 



The plants Atriplex vesicarium, Rhagodia Gaudichaudiana, and Kochia 

 sedifolia have a thick covering of vesicles or air cells. The vesicles of Atriplex 

 and Rhagodia contained little water at the time of our visit, as comparison with 

 the turgid hairs shows ; in Kochia the terminal cells contain air only. We have, 

 therefore, a very efficient air jacket surrounding the whole of the leaf surface. 

 Hence, near the epidermis we have a set of conditions which are constant and 

 unaffected by external factors. Under these conditions the plant transpires 

 regularly, equal amounts of water in equal times, since only the water vapor 

 which gradually diffuses to the surface of the hairs comes under the influence of 

 the external factors and is swept away. This explains the straight-line transpira- 

 tion curve obtained in these plants. The greater insulation of Kochia with its 

 thick felt of hairs is shown in the greater length of curve parallel to the time 

 axis. The fall during the night is ascribed to the more or less complete closure 

 of the stomates and the variations from the curves for the external factors (for 

 example, the second maximum exhibited in most cases from 8 to 9 o'clock) to 

 the regulation of the transpiration by "physiological" control of the stomates. 

 It must be admitted that the term physiological control has been used by many 

 writers as a cloak to hide our ignorance of some of the factors concerned in the 

 mechanism of stomatal regulation, especially during the night. Since we cannot 

 logically ascribe it to "vital" action of the cells, it is probably due to changes 

 in the turgor of the guard cells produced by the action of enzymes on the carbo- 

 hydrates, or possibly to changes in pressure of the water vapour in the inter- 

 cellular spaces of the leaf ; that is, internal factors which were not determined in 

 this research. 



In conclusion, my thanks are due to Mr. Lisle G. Johnson, owner of Dilkera, 

 for his kindness in affording facilities to conduct this work there, and especially 

 to Professor T. G. B. Osborn for his constant help and encouragement which 

 made possible the progress of this research. 



Summary. 

 L The transpiration rates of Casuarina lepidophloia, Geijera parviflora^ 

 Pholidia scoparia, Rhagodia Gaudichatidiana, Atriplex vesicarinfn, and Kochia 

 sedifolia are measured and discussed in relation to the exterhal factors influencing 

 transpiration. 



