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



theless indicate trends of opening and closure of stomata (Heath, 1959). 

 Results obtained in the present study indicate that, for a range of plants of 

 sand dunes, Lloyd's strips provide a useful, if not absolute, index of stomatal 

 aperture. Several strips of the lower epidermis from at least two leaves were 

 taken on sampling occasions ; mean apertures are based on measurements of 

 not less than 20 stomata (frequently more), at least two strips being 

 examined. In view of the importance of starch/sugar changes in stomatal 

 behaviour, at the same time as apertures were measured assessments were 

 made of the amount of starch in the guard cells by the method of Yemm 

 and Wilhs (1954), a scale of 0-5 for starch content being used. 



Infiltration methods integrate the effects of large numbers of stomata 

 and give quick information of stomatal apertures. A variety of infiltrating 

 liquids has been used by different workers (cf Heath, 1959) ; in the present 

 study a graded series of seven solutions involving petrol ether and alcohol 

 was employed. The series was as foUows: i, petrol ether; 2, 4 vols, petrol 

 ether/ 1 vol. absolute ethanol; 3, 2 vols, petrol ether/ 1 vol. ethanol; 4, equal 

 vols, petrol ether and ethanol; 5, i vol. petrol ether/4 vols, ethanol; 6, abs. 

 ethanol; 7, 95% ethanol. Of these solutions, no. i is the most penetrating, 

 and 95 % ethanol the least, entering the leaf only when the stomata are very 

 widely open. To aid the subjective assessment, by eye, of the degree of 

 penetration, crystal violet (cf WilHams, 1949) was added to solutions 3-7 

 [c. 100 mg/150 ml), and waxohne blue to solutions i and 2. Estimations 

 were made by immersing leaves in a shallow layer of the solutions, washing 

 quickly in water and assessing the extent of infiltration. By use of a number 

 of leaves it is possible to find the point in the series (the infiltration index) 

 giving a mean value of 50% penetration of the leaves; trends of stomatal 

 movement can be detected from changes in this index. Parallel studies by 

 the epidermal strip and the infiltration methods gave results which were 

 strongly correlated; for a particular species, a point on the infiltration scale 

 can be expressed in terms of mean stomatal aperture. Some indication of 

 the relation between aperture and infiltration is given for Senecio jacobaea 

 in Fig. 2. However, in view of differences in the structure of stomata (e.g. 

 the presence or absence of ridges or 'beaks' on the guard cells) in different 

 species, it is necessary to determine for each species the mean stomatal 

 aperture corresponding to a particular point on the infdtration scale. 

 Although the infiltration technique provides a rapid indication of stomatal 

 aperture, two main difficulties may be noted. The method is not suitable 

 for use with thickly hairy leaves (e.g. Verbascum thapsus L., Cynoghssum 

 officinale), as the infiltrating solutions may be trapped in the tomentum 

 and hence the degree of penetration is not easily assessed. A second diffi- 



