66 SEEDS AND SEEDLINGS 



with the micropyle in the seed. The thick outer coat (the testa) 

 is easily removed from a soaked bean, the delicate coat under it 

 easily escaping notice. The seed separates into two parts ; these are 

 called the cotyledons. If you pull apart the cotyledons very care- 

 fully, you find certain other structures between them. The rod- 

 like part is called the hypocotyl (meaning under the cotyledons). 

 This will later form the root (and part of the stem) of the young 

 bean plant. The first true leaves, very tiny structures, are folded 

 together between the cotyledons. That part of the plant above 

 the cotyledons is known as the plumule or epicotyl (meaning above 

 the cotyledons). All the parts of the seed within the seed coats 

 together form the embryo or young plant. A bean seed contains, 

 then, a tiny plant tucked away between the cotyledons and pro- 

 tected by a tough coat. 



Food in the Cotyledons. The problem now before us is to find 

 out how the embryo of the bean is adapted to grow into an adult 

 plant. Up to this stage of its existence it has had the advantage 

 of food and protection from the parent plant. Now it must begin 

 the battle of life alone. We shall find in all our work with plants 

 and animals that the problem of food supply is always the most 

 important problem to be solved by the growing organism. Let 

 us see if the embryo is able to get a start in life (which many ani- 

 mals get in the egg) from food provided for it within its own body. 

 Test for Starch. If we shake up a piece 

 of laundry starch in water, in a test tube, 

 and then add to the mixture two or three 

 drops of iodine solution, 1 we find that the 

 particles of starch in the test tube turn pur- 

 ple or deep blue. It has been discovered by 

 experiment that starch, and no other known 

 substance, will be turned purple or dark blue. 



Starch grains in the cells 



of a potato tuber. Therefore, iodine solution has come to be 



used as a test for the presence of starch. 

 Starch in the Bean. If we mash up a little piece of a bean cotyle- 



1 Iodine solution is made by simply adding a few crystals of the element iodine 

 to 95 per cent alcohol ; or, better, take by weight 1 gram of iodine crystals, f 

 gram of iodide of potassium, and dilute to a dark brown color in weak alcohol (35 

 per cent) or distilled water. 



