INTRODUCTION TO PLANT ECOLOGY 5 
The intensity and duration of light varies with the latitude. 
Observation seems to show that plants are even more affected 
by the duration of light than by its intensity. Tropical plants 
are exposed to a more regular succession of equal periods of 
light and darkness than temperate or arctic plants. On the 
other hand, in arctic regions plants, although they have light 
for a shorter period of the year, have it more continuously. 
It is stated that in Finland barley ripens in eighty-nine days, 
whilst a little further south it takes a hundred days, although 
the rays of light are stronger; but in Finland the days are 
_ longer, and the barley is therefore exposed to longer light per 
day, and this duration of light more than counterbalances the 
difference in the strength of the light. 
Reva Gt There is a constant struggle for life going on 
Plants to among plants as among animals. Unless a plant 
Se can adapt itself to the external conditions around 
it, which are constantly varying owing to the 
action of man and the forces of Nature, it must die. It is 
well known, for instance, that plants which live entirely 
submerged in water cannot live out of water; they cannot 
adapt themselves to atmospheric conditions, and therefore 
die. It is found that the vegetative organs of a plant, 
especially the leaf, are those that have this power of adapt- 
ability, whilst the flower is hardly at all affected. This 
is the reason that plants belonging to very different Natural 
Orders will be found associated together in the same district, 
and will tend to resemble each other as far as the root, stem, 
or leaf is concerned, whilst the structure of the flower is widely 
different. This power of adaptability is well seen in plants 
that grow by the sea. The leaves of the same species growing 
inland will be far less thick and succulent than those of plants 
growing by the sea. This may easily be observed in Lady’s- 
fingers (Anthyllis vulneraria), Scurvy-grass (Cochlearia officinalis), 
and many others. It is interesting to note, too, that plants 
may attain the same purpose by different methods. Plants that 
grow on dry, rocky soils have to protect themselves against 
too rapid transpiration, and they may do this in a variety of 
