BY SELF-ADAPTATION TO THE ENVIRONMENT, 245 
superior face, now become the lower in position. Another inter- 
esting point is that in young leaves the mesophyll is homo- 
geneous; but the cells under the upper epidermis are rather 
Jarger at first in the older leaves, showing an hereditary tendency 
to develop palisade-tissue; but later on the influence of the 
light soon renders the cells below the—now reversed—inferior 
side much longer than the others. 
The needle-like leaves of the Norway Spruce prove, according 
to the careful investigations of M. Mer *, to be most remarkab.y 
sensitive to light—becoming more quadrilateral with a uniform 
palisade-tissue as the leaf grows exposed, but flatter and more 
dorsi-ventral when it is more shaded on the same tree. 
He also observes the remarkable result of frost setting up a 
“habit” in the tree, as follows :—“ Trees planted isolated have 
some modifications. They become more bushy, from the deve- 
lopment of lateral buds, which are arrested in forests. When 
the young shoots are struck by frosts in the spring it happens at 
times that, without reaching the point of death, they lose their 
turgescence. They become soft, and their extremity turns 
towards the ground. Lignification supervenes before they have 
recovered their turgescence, and they remain thus definitely 
curved. When the terminal bud is not destroyed it develops 
its succeeding shoot the following year in this position. If 
one places it vertically, the terminal bud turns downwards, and 
the branch in course of development preserves this situation ; 
or rather its extremity tends to elevate itself by a slight 
curvature. This depends upon its degree of vigour and the 
time during growth when the experiment is made.” 
‘This passage is very suggestive as a cause of “ weeping” 
varieties of trees, by a temporary injury producing a permanent 
effect in the growth, though not to the extent of being heredi- 
tary. 
Again, Dr. F. Nollt has shown that external influences 
determine not only the direction of some organs, but also the 
position in which they are formed ; as, e. g., the development of 
the gemma of Varchantia, of aerial roots on climbing plants, 
&e. In other and more numerous cases the formation of fresh 
organs appears to be independent of external forces, and to 
be determined only by the internal, ¢. e. hereditary, forces in the 
* Bull. Soc. Bot. de Fr. 1883, p. 40. 
+ See Journ. Roy. Micr, Soc. 1891, p. 490. 
