396 Ewart . — The Action of Cold and of 
mined, beyond which any increase in the light reaching 
the living cells injuriously affects them, it is possible to calcu- 
late the intensity of the illumination to which the plant 
as a whole may in nature be exposed without any particular 
cells forming part of it being directly injured, provided the 
amount lost by the absorption and reflection of the interposed 
protecting layers, &c., is known. For an account of the 
adaptive modifications shown by the tropical plants, and the 
means by which those growing in exposed situations are able 
to protect themselves from the effects of over-exposure, refer- 
ence may be made to the paper on the effects of tropical 
insolation 1 . 
Here ; except where otherwise mentioned, the thermal and 
photo-chemical effects were allowed to act together, but even 
then it was found that the leaves of most of the plants 
examined were resistant to even prolonged exposure to 
continuous direct sunlight, although slight injurious effects 
might be produced upon them. Naturally, if but one surface 
is directly exposed, only the cells immediately beneath this 
surface become perceptibly affected. By using a mirror, 
however, the under surface of the leaf may be illuminated 
as well, and the cells in the interior subjected to an intensity 
of illumination approaching more closely to that of direct 
sunlight. Many shade-plants are comparatively sensitive to 
prolonged exposure to direct sunlight. Thus the chlorophyll- 
grains of Pisonia alba , Selaginella sp. (?), Chara hispid a, 
Spirogyra crassa , &c., may be almost or entirely bleached 
by a single day’s exposure, and although they have 
temporarily lost the power of assimilation, on returning to 
normal conditions they may become green and commence 
to assimilate again in a few days ( 1 . c., pp. 442, 443). Trans- 
1 Ann. of Bot., Sept., 1897, Vol. xi, p. 339. Giltay (Annales du Jardin Bot. 
de Buitenzorg, 1898, Vol. xv, p. 68) has shown that Cassia timorensis in 
diffuse daylight assimilates 6-4 mg. CC) 2 per \ sq. m. of leaf, but when exposed 
to strong sunlight only 5.7 mg. C 0 2 , owing to the erect position which the 
leaflets assume, and hence owing to this protective adaptation the optimal 
intensity of illumination for the assimilation of the plant as a whole is markedly 
reduced. 
