32 LIGHT IN RELATION TO TREE GROWTH. 
TABLE 5.—Comparative thickness of leaf tissues in micromillimeters. 
Thickness. 
| | Palisad 
Se alisade paren- Spon aren- 
Species. Spidenn- | ee eres 
Mame P< 2e ae aS 
Shaded |Mtumined Shaded Ulumined Shaded |Ilumined 
leaves. | leaves. | leaves. | leaves. | leaves. | leaves. 
Sambucus racemosa (elder) ......-.------- 10 20 0. 75 | 39 60 
Tilia europea (linden)..............--.--- 10 20 0 | 38 | 24 44 
Sorbus aucuparia (Mountain ash).......-- 18 23 15 62 40 a4 
Populus tremula (aspen)...............--- 20 24 50 96 at 58 
Betula verrucosa (birch) ......-.......-..- 17 | 22 | 45 80 45 52 
Ee to one of the following European lindens: T. platyphyllos, T. vulgaris, T. 
cordata. 
Some shade-enduring plants, as Owalis, for instance, when exposed 
to strong light become sickly, since they are unable to modify the 
mesophyllum of the leaves. The other extreme is shown by light- 
loving species, such as Peucedanum, which are unable to endure shade. 
Trees occupy an intermediate position between these two extremes. 
They are capable of adapting themselves more or less, especially those 
species whose life tissues may be either chiefly in the form of a 
spongy parenchyma or chiefly in the form of a palisade parenchyma, 
in accordance with the hght intensity. The stronger the parenchyma 
tissues developed, the more intolerant is the species, and vice versa. 
Therefore, by the thickness of the parenchyma of shaded le eaves, espe-. 
elally of the palisade parenchyma, it 1s possible to determine the 
comparative adaptability of species to shading. Suroj (1891), on the 
basis of his experiments along this line, has arranged the different 
tree species in order of their tolerance (beginning with the most tol- 
erant) as follows: 
Taxus baccata (European yew). Alnus incana (hoary alder). 
Abies pectinata (silver fir). Quercus robur pedunculata (peduncu- 
Tilia (linden). late oak). 
Picea (spruce). Frazinus excelsior (European ash). 
Sorbus aucuparia (mountain ash). Pinus strobus (white pine). 
Acer (maple). Betula verrucosa (birch). 
Alnus glutinose (black alder). Populus tremula (aspen-poplar). 
Ulmus effusa (spreading elm). Pinus silvestris (Scotch pine). 
Larig europea (European larch). 
ASSIMILATIVE CAPACITY OF LEAF. 
The assimilative capacity of the leaves furnishes another good basis 
for determining the comparative tolerance of different trees. It has 
already been brought out that there is a difference in the assimilative 
energy of light-needing and shade-enduring species as influenced by 
light, which is due to the inherent difference in the sensitiveness of 
the chloroplasts of the two groups of trees. The leaves of some species 
(shade enduring) apparently do not possess a great assimilative 
lative activity only in intense light. Assimilation means, in every 
species (light needing) are capable of developing the greatest _assimi- 
