TOLERANCE AND INTOLERANCE OF TREE GROWTH. 13 
in the sensitiveness of their chloroplasts. These figures show also, 
in a general way, that a unit (by weight) of leaf substance of broad- 
leaf species assimilates considerably more than the same unit of leaf 
substance of coniferous species. This is probably due to the differ- 
ence in the anatomical structure of the leaves of the broadleaf and 
coniferous species. The extremely small size of the cells of the leaf 
parenchyma in broadleaf species means the presence of a large 
amount of living protoplasm and chloroplasts, and consequently a 
larger amount of living, acting substance within the same space. 
On the basis of these experiments, Lubimenko claims to have 
established the following ‘facts in regard to the photochemical work 
of different forest trees: 
(1) The photochemical work of the green leaf is determined by its 
anatomical structure and the inherent qualities of its chloroplasts. 
(2) The influence of the anatomical structure of the leaf is felt 
mainly in light of medium intensity; the influence of the inherent 
qualities of the chloroplasts, on the other hand, is strongly apparent 
in light of low and high intensities. , 
(3) Chloroplasts of different species are sensitive to light in dif- 
ferent degrees; chloroplasts of shade-enduring species are more sensi- 
tive than chloroplasts of light-needing species. 
(4) Species with more sensitive chloroplasts begin to decompose 
carbon dioxide and reach a maximum of assimilative photochemical 
energy in ight of a much lower intensity than species with less sen-, 
sitive chloroplasts. 
(5) The curve of the photochemical work of a green leaf has a dis-’ 
tinct optimum, which is reached in different species under differ- 
ent intensities of light and is determined by the inherent qualities 
of the chloroplasts. 
(6) It is very probable that the difference in sensitiveness of 
chloroplasts of different species is due to a difference in the absorp- 
tive capacity of the chlorophyll. 
(7) The conception of “ light-loving ” and “ shade-enduring ” trees 
has a real foundation as far as the process of assimilation of carbon 
is concerned. Leaves of shade-enduring species are able to replace 
the carbon dioxide expended in the mere process of respiration in 
much weaker light than are leaves of light-loving species. The opti- 
mum of assimilation in tolerant species lies within the limits of nor- 
mal sunlight, while the optimum of assimilation in light-needing 
species lies beyond those limits. 
GRAFE’S EXPERIMENTS. 
These conclusions are also corroborated by Grafe’s experiments 
carried on at the Institute of Plant Physiology, at Vienna (Wiesner, 
1907: 266-267), though for another purpose. Green leaves of birch 
(a light-demanding species) and of beech (shade enduring) were 
