304 MARGARET S.JARVIS 



in conditions of more severe drought. A method, of the type suggested by- 

 Levitt (1958), was designed to compare the resistance of the species to the 

 development of critical LWDs, in atmospheres of a range of controlled 

 relative humidities, at a standard temperature. (Critical LWD is the 

 maximum LWD from which recovery of turgidity can be made.) Detached 

 shoots of the species were subjected to the standard drying conditions for 

 a standard time. Half the shoots had the cut ends in water, so that water 

 uptake was not restricted : the other half were not supphed with water. 

 Thus the drought resistance of the species can be compared, in conditions 

 representing both situations where soil water is freely available and is 

 unavailable. The critical LWDs, and the relative humidities which induced 

 their development, were determined for the four species. 

 Methods. The drought chambers consisted of polythene bowls, 30 cm 

 square and 12 cm deep, covered with glass sheets. Twenty glass tubes were 

 stuck, at evenly spaced intervals, on to the bottom of each chamber. Up to 

 half of these were fdled with water. The tops of all were sealed with 

 'parafilm'. The relative humidity (RH) was controlled by one htre of an 

 appropriate sulphuric acid-water mixture. The change in this solution 

 during an experiment was never greater than that equivalent to a 1% 

 increase inRH. The experiments were carried out in growth chambers at 

 25°C, with a standard Hght source. 



Twenty leafy shoots were used in each chamber ; half with their stems in 

 water : half without water and with the cut ends of the stems Vasehned. 

 The stems were pushed through the 'parafdm' seals into the glass tubes. 

 The 'parafdm' prevented direct evaporation from the water into the 

 drought chamber; and also supported the stems in positions arranged to 

 give even distribution of the leaves within the chambers. 



The shoots were subjected to the controlledRHs for 20 hours; 12 hours 

 in the dark, followed by 8 hours in the hght. The dark period was designed 

 to allow RH equilibration throughout the chamber, while water loss was 

 restricted by stomatal closure. During the drought period the chambers 

 were rocked gently from time to time to ensure change of the acid-water 

 surface and to promote circulation of the air in the chamber. At the end 

 of the period the acid-water mixtures were siphoned off and replaced with 

 water. Samples of leaves were taken for LWD determinations. The bases 

 of all the shoots were re-cut and replaced in the tubes, which had all been 

 fdled with water. The chambers were placed in the dark for 24 hours, after 

 which period recovery of the leaves was estimated. Recovery of turgidity 

 could be assessed subjectively, or objectively by comparing the ratio of 

 saturated weight (i.e., the weight of the leaves after 24 hours in the dark, 



