THE STRUCTURE OF SEEDS 33 



as corn and other grasses, lilies, palms, etc., two in many common 

 plants like the bean, squash, apple, and buttercup, and many in 

 pines and other evergreens. So important is this difference that 

 all plants that bear seeds are classified as: 



Monocotyledonous (having one cotyledon), 

 Dicotyledonous (having two cotyledons), 

 Polycotyledonous (having three or more cotyledons), 



and can be placed in one of these three divisions, which also agree, 

 as well, in structure of stem, leaf, and flower. 



Plumule. The plumule is that part of the embryo above the 

 cotyledons, from which develops the shoot proper, consisting of 

 stem, leaves, and flowers. It may vary much in size and develop- 

 ment. If much food be stored, either in cotyledons or endosperm, 

 the plumule may be small. On the other hand if little food be 

 provided, the plant must early shift for itself, and so the plumule 

 may have several well-formed leaves, wanting only exposure to 

 light to become a self supporting plant. 



Hypocotyl. The primitive stem, or all that part of the embryo 

 below the cotyledons, is the hypocotyl. From its lower end the 

 root system develops. Upon its upward lengthening depends 

 whether the cotyledons shall emerge from the soil when germination 

 takes place. 



Endosperm. Though the endosperm is usually present at some 

 stage, it is not found in all seeds when they are mature, since it 

 may be entirely absorbed by the growing embryo, its function of 

 food storage being assumed by the cotyledons. It is, however, 

 very important in many seeds, especially the grains. From its 

 store of starch we derive our bread. Food for the embryo may 

 be stored either in the endosperm or cotyledons. Our laboratory 

 tests show that this stored food consists largely of starch, to- 

 gether with considerable proteid, a little fat or oil, and some 

 mineral matter. 



The seed has within itself the miniature plant', or embryo, and 

 all the kinds of nutrients needed for growth except water. This 



