INTRODUCTION 15 



large intercellular spaces which are not so numerous 

 in the palisade cells, and, leading through them, 

 vascular bundles. Around the vascular bundle is 

 a border parenchyma for water conduction. In the 

 epidermis below are stomata with guard cells on 

 each side, closed at night, open in daytime. 



The intercellular spaces in the palisade cells 

 and spongy parenchyma communicate, and carbon 

 dioxide entering by the stomata is thus freely dis- 

 persed. Light can enter the epidermis and is 

 absorbed by the chloroplasts, and under its influence 

 the carbon dioxide entering by the stomata is broken 

 up or rendered available for the fixation of carbon 

 for use by the plant, or the manufacture from carbon 

 dioxide plus water of a carbohydrate having the 

 chemical formula, CgH^gOg. In the process oxygen is 

 given off. Water is present above and below the 

 mesophyll, and is transferred by the border paren- 

 chyma from the veins to the other parts of the leaf. 

 The veins carry the above products away from the 

 palisade and spongy cells. The veins thus bring 

 water and carry away the products formed. 



Leaves also serve, as do stems, to transfer away 

 the superabundant water, and this is done by the 

 stomata and intercellular spaces. 



This water escapes as vapour. Carbon dioxide is 

 also given off by the stomata. Under respiration, 

 as opposed to assimilation, oxygen is utilised by 

 all cells, and carbon dioxide is given off, as in animals. 

 The protection of the stomata from being clogged 



