172 A.J. WILLIS AND R.L.JEFFERIES 



to the behaviour of stomata. For example, stomatal closure from fairly 

 wide apertures (c. 4-5 /x) probably occurred about one hour after excision 

 of the leaf at 1 1 a.m. ; however, in the leaf detached in the early morning, 

 although the stomata were initially of only moderate aperture {c. 3-3 /la), 

 closure was delayed. 



The rate of transpiration at the time of cutting was estimated by extra- 

 polation of the decline curves to zero time. The vaHdity of this estimate 

 may be questioned in view of the quick changes in transpiration rate 

 which may occur on cutting a leaf, in particular when the stomata are 



ii- 



100 



z 



se 



o 

 z 







_j 



z 

 



Q. 



200 



300- 



TIME 



90 

 EXCISION 



Fig. I. Transpiration decline curves for Scncciojacobaea. For simplicity in comparison, 

 transpiration losses are calculated as mg/g initial fresh weight of the excised leaves. 



A selection of the results for leaves cut at 6-27 a.m. (• •), 10-55 a.m. (O O) 



and 6.18 p.m. (a a) on 18 June 1958 is shown. 



only partly open. Substantial increases in transpiration rate on leaf excision 

 have been noted by many observers, including Andersson, Hertz and 

 Rufelt (1954) and Parker (1957) ; similar observations have also been made 

 under some conditions in this laboratory. Milthorpe and Spencer (1957) 

 obtained evidence of increased transpiration rates and opening of stomata 

 following the removal of the water supply of leaves, and further showed 

 that data of Hygen (1953) are consistent with these findings. However, 

 there is only httle indication of appreciable increases in transpiration rate 

 on leaf excision in the present field studies ; in fact, the points of the majority 



