14 



Miss E. K. Saunders. On a Discontinuous 



Stachys palustris. Vesque and Viet,* in a note to the same effect, 

 also draw attention to an experiment made by Kraus upon the 

 etiolated shoots of the potato, in which he found that those formed 

 in the dark are glabrous when grown in damp air, and downy when 

 the atmosphere is more or less dry. 



In the case of Biscutella laevigata, however, it seems impossible to 

 explain the variations which occur as the result of a direct modifica- 

 tion of habit to habitat — to conceive of them as due to differences in 

 temperature, in illumination, in the humidity of the atmosphere, or 

 in the nutritive capacity of the soil. For in all the instances quoted 

 above, a certain constancy of form is associated with, and character- 

 istic of, a particular environment ; whereas in the species in question 

 no such connexion is apparent, the two extreme types are not 

 infrequently found in groups in close proximity to one another, or a 

 hairy and a glabrous plant may even be growing in the same sod of 

 turf, and presumably, therefore, under identical external conditions. 



With the immediate cause of these variations, however, I am not 

 here concerned further; whatever may be the active agent in their 

 production, their occurrence suggested that a detailed study of these 

 differences in the leaf surface might lead to interesting results 

 bearing upon the views which have recently been brought forward 

 with regard to discontinuous variation and its value as a factor in 

 the origin of species. 



Before enteriug upon an analysis of the experimental results it 

 will be necessary to consider somewhat more in detail the variations 

 in the degree of hairiness or smoothness exhibited by the leaf- 

 surface. The following types may conveniently be distinguished : — - 



I. Surface hairy. — In the leaves of this class both the upper and 

 the lower surfaces are thickly covered with hairs. In some 

 individuals the hairy character of the leaf surface is easily 

 recognisable at a glance ; in others, in which the hairs are 

 short, a closer inspection is necessary. 

 II. Surface intermediate. — To this type belong — 



(a) Leaves in which a few stray hairs are scattered thinly 



over the upper surface. 

 (6) Leaves in which (more frequently) some portion of the 

 upper surface is quite glabrous. This smooth area 

 usually forms a longitudinal zone of varying breadth 

 on either side of the midrib ; the hairs, indeed, may 

 be restricted to the margin and to a narrow belt of 

 marginal surface. In rare cases the hairs are grouped 

 differently : thus, the apical region may be hairy while 

 the basal and middle portions are glabrous. 



* ' Ann. des Sci. Nat.,' Ser. VI, t. 12, footnote to p. 174. 



