150 The Irish Naturalist. June, 



epidermis. As a rule, at the depressed spots the cells of the 

 palisade parenchyma are much shortened (so much so in some 

 cases that one can hardly speak of palisade parenchyma at all), 

 the leaf is considerably thinner, and the contrast between 

 the structure of the spot and that of the surrounding tissue 

 strongly reminds one of that between a "sun-leaf" and a 

 *' shade-leaf" of one and the same plant. The cells of the 

 leaf at these depressed spots seem to be rather richer in 

 protoplasm, and to contain rather larger nuclei than the cells of 

 the green part of the leaf ^ As regards the distribution of the 

 stomata, I counted these over the flat spots and over corres- 

 ponding green portions near the same spots. The average 

 number of stomata on an area of 0*2 sq. mm. was as follows : — 



spot. green. 



Leaf A. I Upper Side, 1-40 i-66 



'^ Lower „ 4-56 5*26 



LeafB. | ^pper „ 2-93 2-92 



'^ Lower „ 9*64 9-00 



From these numbers it will be seen that there is no relative 

 increase in the number of stomata on the spotted areas. Stahl 

 found that the ends of the leaves in Setnpervivum tectoriim where 

 there is an increase in the number of stomata were specially 

 characterised by the red colour, whereas in Medicago intertexta 

 there were fewer stomata on the spots than on the green parts. 



Concerning the amount of sugar present in the different parts 

 of the spotted leaves (starch is not formed apparently in these 

 leaves at all), I have not succeeded in determining whether the 

 spots contain more or less sugar than the normal green parts 

 of the leaf, as Fehling's solution rapidly causes maceration of 

 the tissues, and hence the escape of the sugar. I would, how- 

 ever, suggest to an}^ one who is interested in this part of the 

 subject, and who has a good supply of spotted leaves con- 

 venient for use, that the spots might be carefully cutout of a 

 number of leaves, weighed, and put into warm distilled water 



' I do not quite understand Mr. Colgan when he speaks (p. 8j) of the 

 cause of these depressed spots h^\\\% physical rather than physiological. 

 Possibly he supposes that the leaf behaves as a thin sheet of metal with 

 a dark spot on it might be expected to do on exposure to radiant heat, 

 namely, that the warmer dark part would tend to bulge owing to unequal 

 expansion. I have discovered no signs of the results of " a system of 

 strains " such as he postulates in the tissues of these spots. 



