10 PLANT GROWTH 



castor-bean, Iris (Fig. 3) and corn (Fig. 1), the food is 

 located in a region called the endosperm, which is the prod- 

 uct of the polar bodies of the ovule and the second sperm. 

 In exalbuminous seeds, such as the common bean, the food 

 is stored in the two cotyledons ( Fig. 2 ) . These fleshy bodies, 

 readily distinguishable in cooked beans, are the first struc- 

 tures to appear above the ground in germination. 



Plants are classified according to the number of cotyledons 

 or seed leaves of the embryo. Those growing from seeds 

 with one cotyledon, such as corn, are called monocotyledons; 

 those growing from seeds with two cotyledons are called 

 dicotyledons. The cone-bearing trees, such as pines, bear 

 seeds with more cotyledons, usually from five to eleven. 



Monocotyledonous seeds of the grass type have a small 

 embryo and a large endosperm. The embryo is only about 

 8 per cent of the wheat grain, but it is a little larger in corn. 

 The epithelium (shown as a double line in Fig. 1), is a single 

 layer of cells between the embryo and the endosperm, which 

 secretes enzymes to digest the stored food when conditions 

 are favorable for germination. The iris type has a small 

 embryo (Fig. 3, C) embedded in the endosperm. Such 

 monocotyledonous seeds as the grasses and orchids are so 

 small that their structures are difficult to see and their germi- 

 nation is poor because the supply of stored food is so limited. 



The best seeds should always be used by the gardener, 

 because the difference in cost is so small when compared with 

 the difference in the plants grown from the best and from 

 inferior seeds. Select well-bred seeds of good varieties from 

 reliable seedsmen. Home-collected seeds of many species 

 produce inferior plants, largely because of cross pollination. 

 For those who are interested in developing their own seeds 

 suggestions for artificial pollination are described in Chapter 

 18. 



The vitality or viability of a seed, which is its internal 

 capacity to germinate and grow under favorable conditions, 



