1903- PE'Thybridge. — The Ledf-spots of Aruvi maculahim. 149 



Further observations on similar lines are highly desirable. 



With regard to the form of the spots, I have observed both 

 the plain flat spots and those in which a pseudo-blister is pro- 

 duced. This bulging of the spot is more often downward than 

 upward, although on one and the same leaf one may some- 

 times notice the spots both as depressions and as elevations. 

 I have not so far succeeded in finding satisfactory transitions 

 from the flat to the repousse form of spot ; as a rule, either all 

 the spots on a given leaf are flat or all are hollowed, although 

 some of the smaller spots on the leaves with depressed spots 

 are sometimes practically flat. 



It is to be noted that in these pseudo-blisters the colouring 

 matter shows as distinctly on the lower surface, or nearly so, 

 as on the upper. I have examined microscopically sections 

 through the spots of a fairly large number of leaves and have 

 never found any trace of insect attack or fungus hypha. The 

 structure of the leaf at the spots differs, in some cases quite 

 considerably, from that of the unspotted portions. In the case 

 of the flat spots the leaf is nearly always slightly thinner at 

 the spot ; this is due chiefly to the shorter length of the cells 

 of the palisade parenchyma, and it is these cells and these 

 alone which in this case contain the red colouring matter, 

 hence it does not show through to the lower surface of the 

 leaf. Stahl ^ mentions that he found the anthocyan localised 

 in Arum in the spongy parenchyma of the leaf, but does not 

 mention whether the spots were flat or whether they formed 

 depressions. Correlated with the shorter length of the 

 palisade cells of the flat spots is the fact that the whole 

 tissue of the leaf here is slightly looser in texture, thereby 

 causing a rather larger development of the intercellular spaces. 

 This latter point is perhaps more clearly seen in the depressed 

 spots. On cutting sections of fresh leaves the air imprisoned 

 in these spaces produces a quite appreciably deeper blackness 

 at the spot than in the normally green parts of the leaf 

 bordering on it. The anthocyan at the depressed spots is 

 more abundant than at the flat ones ; it is often distributed 

 both in the palisade parenchyma and in the spongy tissue of 

 the leaf, but is sometimes confined to the latter. I have not 

 observed it in any case in either the upper or the lower 



■• 3tahl, loc, cit.^ p. 182. 



