140 THE PLANT 
4 
to the excessive elongation of the parenchyma cells, but the cause of the 
latter is far from apparent. It is generally assumed to be due to a lack of 
the tonic action of sunlight, which brings about a retardation of growth in 
Fig. 40. Isophotophyll of AJlionia linearis, 
showing diphotic ecads: 1, light 1; 2, light .012; 
3, light 003. X< 130, 
slight development of hairs in sciophilous 
sun plants. The evidence in 
favor of this view is not con- 
clusive, and it seems probable 
at least that the elongation of 
the parenchyma cells takes 
place under conditions which 
favor the mechanical stretch- 
ing of the cell wall, but inhibit 
the proper growth of the wall 
by intussusception. It is 
hardly necessary to state that 
the reduced photosynthetic ac- 
tivity of shade plants favors 
such an explanation. What- 
ever the cause, the advantage 
that results from the elongation 
of the internodes is apparent. 
Leaves interfere less with the 
illumination of those below 
them, and the leaves of the 
branches are carried away 
from the stem in such a way as 
to give the plant the best possi- 
ble exposure for its aggregate 
leaf surface. 
179. Modification of the 
epidermis. The development 
of epidermal chloroplasts in 
diffuse light is the only change 
which is due to the direct effect 
of light. This does not often 
occur in the shade ecads of 
sun species, but chloroplasts 
are regularly present in the 
epidermis of woodland ferns 
and of submerged plants. The 
plants is an advantage, but it 
must be referred to the factors that determine water loss. The significance’ 
