68 BRITISH PLANTS 
in summer they revolve in the full light of the sun, and 
there is no night. The long duration of the summer 
days in Northern regions compensates, in some measure, 
for the shortness of the summer and the low intensity 
of solar radiation. In Norway, for example, cereals 
ripen in a much shorter time than in England. In the 
Tropics there is no alternation of summer and winter, 
because the sun is never far from the zenith at noon ; 
the vegetation is therefore always green, and, provided 
sufficient water is present, does not display the seasonal 
changes of activity and rest so familiar to us. 
Heliotropism.—The growing parts of plants are affected 
by certain physical forces in the environment, with the 
result that the different organs tend to turn towards a 
position of advantage and away from a position of dis- 
advantage. Curvatures of this kind are known as 
tropisms. Perhaps the most obvious of these tropisms is 
that which is brought about by oblique illumination. 
Stems grow towards the light, and the leaves bend so as 
to receive as much of it as possible. The heliotropism 
(Gr. helios, sun ; tropos, turning) of stems and leaves is 
easily observed in plants grown before a window in a 
dwelling-house. The same thing happens outside. 
Plants growing in shady situations bend towards the 
direction in which they can catch the greatest amount 
of light. The stems curve towards a position of advan- 
tage, and here the position of advantage is that best 
suited for assimilation. 
Leaf - Mosaic.—Closely associated with heliotropism is 
the phenomenon of leaf-mosaic. If the leaves of a horse- 
chestnut be viewed from above, it will be found that they 
are so arranged in regard to size and position that each 
leaf gets its full share of the light without let or hindrance 
from its fellows. On looking down upon the leaf-surface 
of a shoot there is little or no overlapping, and each 
portion of its area is occupied by a leaf. This arrangement 
is brought about by the bending of the leaf-stalks under 
heliotropic influences, and by variation in the length of 
the petioles and the size of the leaves—e.g., climbing plants 
like ivy (Figs. 20 and 21), prostrate stems like ground-ivy, 
rosette-plants like dandelion, and bushes with oblique 
or horizontal stems like the box, yew, and maple (Figs. 22 
and 23). 
