46 THE STORY OF PLANT LIFE 



placed in the light and disperse if light is withdrawn, 

 showing oxygen is given off through assimilation 

 only in the light. Seen through the spectroscope 

 the bacteria collect mostly under the red light, less 

 so under the orange and yellow rays. 



The border parenchyma brings water from the 

 veins to the palisade and spongy parenchyma, and 

 food is transferred from them to the sieve tubes to 

 travel down the leaf. 



The intercellular spaces provide channels for the 

 intake of gases. The epidermis lets light into the 

 top layer of palisade cells where lie the chloroplasts, 

 arranged along the walls vertically, with intercellular 

 spaces between with carbon dioxide and water. Two 

 per cent, of the light passing through the leaf is used 

 in synthesis, the rest in the evaporating of the water 

 into water vapour. The border parenchyma cells 

 thus bring water and take back food from the 

 palisade and spongy cells, and the phloem and veins 

 carry it down to the stem and elsewhere. 



In order that photo-synthesis and carbon fixation 

 may occur there must be proper light, the stomata 

 must open and admit carbon dioxide, the veins must 

 bring the transpiration water from below. They 

 must also carry down the food, as made or 

 elaborated. 



The formation of starch takes over an hour in 

 some cases, five minutes in others. 



The proportion of carbon dioxide in the atmosphere 

 is very small — '03 — but this is sufficient, under the 



