8o MINUTE MARVELS OF NATURE 



openings, which can distinctly be seen around 

 the edge of the leaf section as light-coloured 

 slits amongst the dark external tissue. And it 

 is by means of these mouths that the inter- 

 change of gases takes place. They open during 

 sunlight and close during darkness ; and it 

 is estimated that there are in one square 

 inch of the underside of a lilac leaf 160,000 

 mouths, where, by way of contrast, in the same 

 space on the mistletoe leaf only 200 are found. 

 But it must be remembered that the mistletoe 

 is a semi-parasitic plant, and therefore does not 

 altogether earn its own living. 



Pig. 52 shows a portion of the skin tissue from 

 the leaf of a tulip and presents a good illustration 

 of the manner in which the epidermal cells are 

 arranged. In the centre of each cell will be seen 

 the nucleus or life-centre, while amongst the cells 

 the little mouths or "stomata," as the botanist 

 terms them, show plainly. 



Through these numerous tiny mouths, then, the 

 carbon dioxide is absorbed by the leaves, and is 

 thence passed to the green chlorophyll corpuscles 

 in the palisade cells for them to perform their 

 most important function of freeing the oxygen 

 and retaining the carbon. To these little green 

 atoms, in fact, we are indebted for the oxygen 

 without which life would cease to exist. But this 



