444 MR. A. J. EWART ON ASSIMILATORY INHIBITION. 
position. The more delicate young leaves are very commonly 
protected either by being shaded by older leaves, or by a hanging 
position, or by the formation of a red dye. In tropical plants, 
where the exposure is much more intense, the abundance and 
importance of the protective agencies is correspondingly in- 
creased. In many tropical plants the leaves place themselves 
obliquely to the sun’s most intense perpendicular rays; but 
most common of all is the formation of a red dye in either the 
epidermal cells or in the mesophyll cells as well. Johow * has 
shown that the formation of the red dye is directly due to the 
action of light, the exposure acting as a stimulus on certain 
leaves, and causing in them a formation of red dye. In very 
many cases this irritability disappears as the leaf becomes adult, 
and the formation of the dye having ceased, that which is already 
present in the cell is soon decomposed or absorbed. Engelmann + 
has shown that the red dye allows nearly all the rays absorbed 
by chlorophyll to pass through, cutting off for the most part 
only the rays which are of little or no use for assimilation. The 
greatest absorption takes place in the yellow and yellow-green, 
and a lesser but still large amount in the blue, the resultant 
light being red. The amount absorbed by the red sap may 
amount to 4 or 3 of the incident light, and it is just those rays 
which, according to Pringsheim, produce the greatest photo- 
chemical effect which are thus removed. 
Keeble supposes that one use of the red dye may be as a 
protective influence against the heating effect of intense sun- 
light. There can be no doubt, however, that transpiration is by 
far the most important agent in keeping the leaf at a low 
temperature; and, as Keeble himself shows, the difference of 
temperature produced by the presence of the dye is very slight, 
and does not amount to more than from 1? C. to 2°C. Pick + 
concludes that certain rays of light hinder the translocation of 
starch, and that the red dye is of importance in shielding the 
leaf from such rays. In support of this contention he argues: 
* Johow, “ Ueber die Beziehungen einiger Eigenschaften der Laubblütter 
zu den Standortsverháltnissen," in Pringsh. Jahrb. Bd. xv. p. 299. 
t Engelmann, * Die Farben bunter Laubblätter und ihre Bedeutung für 
die Zerlegung der Kohlensäure im Lichte,” in Bot. Zeit. 1887, Nos. 27 & 28. 
1 Pick, * Ueber die Bedeutung des rothen Farbstoffes bei den Phanerogamen 
und die Beziehungen desselben zur Stárkewanderuug," in Bot. Centralblatt, 
xvi. pp. 9-12. 
