i8o A.J. WILLIS AND R.L.JEFFERIES 



20% by midday. The deficit was subsequently reduced, a feature doubtless 

 related to the closing of the stomata and a reduced transpiration rate. 



As discussed with reference to Senecio jacohaea, there is seen to be here 

 also evidence of stomatal regulation of transpiration ; the daily course of 

 transpiration appears to be rather similar in these plants. Rates of 'cuticular' 

 transpiration are also of about the same magnitude (o-y-o-p mg/g/min 

 under conditions of high evaporation). Some difference may be shown 

 with respect to the water deficits of leaves ; usually the deficits are more 

 readily restored overnight in CYnoglossum officinale than in Senecio jacohaea. 

 This feature may be related to differences in habit and root systems of the 

 plants used experimentally; in the erect second-year phnts o£ Senecio the 

 root and conducting systems may be inadequate to supply the water 

 required by the numerous leaves, whereas the first-year plants of Cyno- 

 glossmn have fewer leaves in a basal rosette, and an extensive tap-root (cf 

 Sahsbury, 1952). 



(d) Investigations on Hydrocotyk vulgaris 



Well-developed leaves of tliis plant collected from vigorous populations 



in the 'slacks' were used experimentally. 



Some of the observations made on 18 June 1958, and on 16 June 1959, 

 are shown in Fig. 5. Transpiration rates rise from low values in the early 

 morning to fairly high rates by about 8 a.m. These rates remain high for 

 5-7 hours, and then decline to low values in the early evening. The daily 

 course of transpiration of H. vulgaris is similar to that of evaporation. The 

 stomata are almost closed at night, opening during the morning; they 

 stay widely open for some hours and then close. A general parallelism may 

 be noted between the size of the stomatal aperture and transpiration rate ; 

 on 16 June 1959 the stomata closed appreciably after i p.m. and there was 

 a decline in transpiration rate at about this time. On the other hand, on 

 18 June 1958, the stomata did not close so quickly (at 2 p.m. the mean 

 aperture was 3-6 ij) and the transpiration rate was maintained at a high 

 value for longer. The record of water deficit indicates that the leaves are 

 turgid in the early morning, but by midday may develop a deficit of 12% 

 or more. This deficit is, however, to a large extent restored even by dusk. 



General confirmation of these findings is given by a number of other 

 experiments. The results of one of them (on 21 June i960) are shown in 

 Fig. 6. In this investigation the stomata opened in the morning and were of 

 high aperture for much of the day. Typical trends in the starch content of 

 the guard cells were observed; starch decreased during the phase of 

 stomatal opening, was present in only small amounts when the stomata 



