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 



