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



that this record does not indicate any re-opening of the stomata after 

 midday. 



Similar results to those described have been obtained on a number of 

 occasions. Usually the transpiration rate is highest just before midday and 

 declines substantially in the afternoon. Less commonly a fairly high 

 transpiration rate is maintained well into the afternoon; for example, on 

 i8 June 1958, a rate of about 14 mg/g/min persisted for some 6 hours 

 (from about 10 a.m. to 4 p.m.), the stomata showing fairly constant aper- 

 tures of about 4 fjL over this period. 



Characteristically the stomata of 5. jacobaea show pronounced diurnal 

 changes; shut during the hours of darkness, they open early in the morning 

 to a maximum before midday. They then tend to close with various 

 degrees of rapidity. On one occasion there was some indication of midday 

 closure of stomata, followed by sHght re-opening later in the afternoon, as 

 reported for various plants by Loftfield (1921) and Sayre (1926), but usually 

 no re-opening is observed. 



Water deficits greater than 20% have been found several times, and 

 during the day in the summer are usually well in excess of 10%. Often the 

 deficit developed during the day is still quite large on the following morn- 

 ing, especially at times when the water content of the sand is low. Despite 

 the fact that the water deficit of the leaves may be 8-10% at dawn, the 

 stomata usually show a strong opening reaction and closure often does not 

 begin until the deficit reaches about 15%. 



Attempts have been made to estimate the rate of 'cuticular' transpiration 

 from decline curves; under field conditions the final portion of the curves 

 is not strictly rectilinear and the assessments are to some extent arbitrary. 

 The results indicate a rather low value of 0-6-0-8 mg/g/min under condi- 

 tions of high evaporation. These values are of the order of 5-8 % of the 

 total transpiration losses. 



(c) Observations on Cynoglossum officinale 



The water relations of this plant show a general similarity to those of 

 Senecio jacobaea, with which it is often associated on the dry dune slopes at 

 Braunton Burro ws. Leaves from first year rosettes were used experimentally . 

 A representative set of data is given in Fig. 4. The transpiration record 

 indicates a gradually rising rate to a maximum value at about 10 a.m., soon 

 followed by a rather sharp decline. On the other hand, the rate of evapora- 

 tion reached a high value by 10 a.m. but remained high for some hours. It 

 is clear that the peak transpiration rate precedes the maximum light inten- 

 sity, maximum temperature and lowest relative humidity. In this experi- 



