74 Keehle . — 77z^ Hanging Foliage of 
leaf, conversely, absorbs more of the sun’s thermal rays than 
the red. Now, the two sets of leaves differ in two respects : 
first, in colour ; then in that the green has a thicker, more 
developed cuticle (and mesophyll) than the red. This might, 
therefore, be expected to oppose a more solid resistance to 
the heat-rays than the thin immature cuticle of the red leaf. 
That such is not the case must be regarded as due to the fact 
that the different colouring-matters have different powers of 
reflection and absorption of heat, and that this difference is of 
such a nature that the red colouring- matter more effectually 
cuts off heat-rays from the body of the leaf than does the 
green. That is to say, the red colouring-matter acts as 
a screen by which the thermal effects of the sun’s rays are 
moderated. That the red colouring-matters of leaves act 
powerfully in protecting the leaf from too intense action of 
light has been recently the subject of a research by Pick. 
This investigator finds that the photometric properties of the 
red colouring-matter enable it to protect the leaf, especially 
by cutting off rays which would interfere with the transloca- 
tion of carbohydrates h 
In addition, then, to its value as a screen, the red or reddish 
coloured saps of such trees as Amherstia nobilis have the 
capability of affording to the young leaf, if necessary, a pro- 
tection against too great heating effects of the rays of 
a tropical sun. 
This colouration of the young foliage of trees of low 
latitudes is of very general occurrence. So general is it that 
a tropical forest presents at the time of its leaf-renewal an 
appearance which for beauty of tints rivals that which the 
trees of temperate climates exhibit in early autumn 2 . 
It may be that these coloured saps, occurring often in trees 
whose leaves are, from their youngest stages, liable to a full 
1 Pick, Bot. Centralblatt, B. XVI, Nos. 9-12. 
2 After writing the above I find the following remark in the paper by Johow to 
which reference has already been made, which shows that the prevalence of red 
saps in young foliage is not underestimated by me : ‘ All at once ’ (he is writing of 
the Lesser Antilles) ‘ a red tint, due to the young foliage of the trees, appears in the 
landscape.’ Johow, loc. cit. p. 295. 
