SEEDLINGS 



stomata, and their two guard cells can close together when 

 it is best for the plant. Very little water can pass through 

 the outer cell walls of the epidermis, but when the stomata 

 are open, water given off by any cells within the leaf can 

 pass through the stomata into the air. 



Notice that the upper side of the castor-oil leaf is a 

 darker green than the lower. This is because the cells here 

 are packed very closely together, and also because they 

 have a good supply of chlorophyll. The sun shines directly 

 on them during the warmest part of the day. These cells, 

 then, are the workshops, or laboratories, where the food- 

 making begins. The most important material for making 

 organic food is carbonic acid gas. This is taken directly 

 from the air by the cells of the epidermis, and is passed on 

 through their walls to the laboratory cells. The rest of the 

 raw material is brought by the woody fibres. Just how 

 this crude sap is drawn up, is not easy to explain. The 

 sun helps to raise it, and the eager root-hairs and other 

 cells force it up, but not even the wisest students profess to 

 know all about it. 



Not nearly all of the water brought to the leaves is 

 used by them. Shut up some of your seedlings in a fruit 

 jar, and in a few moments moisture will be seen on the 

 sides of the jar. This is because the leaves are constantly 

 giving off moisture through their stomata. That is, there 

 is a constant current of water streaming up through plants 

 by way of the woody tissue, and much of the water is used 

 simply to carry the other materials for food-making from 

 the root-hairs to the laboratory cells. 



In manufacturing food, the protoplasm, helped by the 

 sunlight and chlorophyll, breaks up carbonic acid and gives 

 off oxygen. The oxygen escapes through the stomata, and 

 you can see bubbles of it if you put plants under water in 

 the sunlight. But this is only a small part of the story of 

 what goes on in the laboratory cells of plants. This first 



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