PLANT GROWTH AND NUTRITION 



59 



formed in the seed as the result of the fertilization of the egg cell, 

 is able to grow into an adult plant. Two sets of factors are neces- 

 sary for its growth : first, the presence of food to give the young 

 plant a start ; second, certain stimulating factors outside the young 

 plant, such as water and heat. 



If we open a bean pod, we find the seeds lying along one edge of 

 the pod, each attached by a little. stalk to the inner wall of the 

 ovary. If we pull a single bean from its attachment, we find that 

 the stalk leaves a scar on the 

 coat of the bean ; this scar is 

 called the hilum. The tiny 

 hole near the hilum is called 

 the micropyle. Turn back to 

 the figure (page 54) showing 

 the ovule in the ovary. Find 

 there the little hole through 

 which the pollen tube reached 

 the embryo sac. This hole is 

 identical with the micropyle 

 in the seed. The thick outer 

 coat (the testa) is easily re- 

 moved 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 coty- 

 ledons very carefully, you find certain other structures between 

 them. The rodlike 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 protected by a 

 tough coat. 



Three views of a kidney bean, the lower 

 one having one cotyledon removed to 

 show the hypocotyl and plumule. 



