66 PLANT LIFE 
whether into or out of its interior, is achieved 
as the result of a nice adjustment to the 
physical conditions that regulate the diffusion 
of gases through a perforate membrane. If 
we try to explain to ourselves how such a 
mechanism could have become so perfectly 
evolved, how the correlation between the cells 
of the epidermal tissue became so perfectly 
—and apparently so purposefully—arranged 
and adjusted, we shall find ourselves con- 
fronted with a task of no mean order. And 
the same difficulty arises whenever we attempt 
to give a satisfactory explanation of any 
other instance of complex adaptedness in the 
structure of living things. 
Utilising the physical advantages which 
the arrangement of its constituent cells and 
tissues have placed at its disposal, the oak 
leaf, under the influence of light from the 
sun, of carbon dioxide from the air, and of 
water from the soil, carries on the operation 
of photosynthesis in certain cells which are 
situated just beneath the epidermis. From 
their form these are commonly known as 
** palisade cells,” and they are continuously 
active, provided the general conditions, such 
as suitable temperature, light, and adequate 
supplies of oxygen and of carbon dioxide, 
are fulfilled. The need of oxygen by plants, 
in contrast to animals, is a very modest one, 
and indeed the oxygen which is liberated 
within the leaf during the process of photo- 
synthesis may really suffice for respiratory 
