4 



PLANT LIFE 



light. As this young stem extends in length it either forces the 

 first seed leaves upward and toward the light or raises the seed itself. 

 If the seed is in the ground the lengthening of the young stem 

 may either push the young leaves up out of the ground, or push 

 the remainder of the seed into the air. This is called sprouting. 

 The seed leaves are called cotyledons, and the young growing 

 stem is called the hypocotyl. The next 

 leaves formed, and which appear in the 

 growth after or above the seed leaves, form 

 the plumule. This is the terminal bud of 

 the young plant. 



Depth of Planting. Seeds differ in the 

 character of growth of the hypocotyl. In 

 the field and garden pea, for example, the 

 two halves of the seed, called cotyledons, 

 are usually not forced above the ground. 

 In the germination of melons, squash, 

 beans and others they are lifted through 

 the surface of the ground into the air. 



Because of these differences in seeds the 

 depth of planting must be varied. Those 

 seeds which push the cotyledons out of the 

 ground should not be planted so deep and 

 care must be exercised to avoid the soil 

 becoming crusty at that time. Those which 

 leave the cotyledons in the ground may 

 use their energy in forcing the ascending 

 stem and leaf cluster through a firmer 

 layer of soil. Peas are usually planted much deeper than beans. 

 Depth of planting is somewhat in proportion to the size and 

 vigor of the seed. The character and amount of moisture in the 

 soil also help to govern the depth of planting. For any given kind 

 of seed, plant deeper in warm weather and in light soils. Plant 

 shallower in wet weather and in heavy soils. 



THE LEAVES 



Green Color. As soon as the plant appears above ground 

 light causes it to assume a green color. This is because the proto- 

 plasm or life fluid of the plant produces a substance called chloro- 

 phyll. Its purpose is to enable the plant to make use of sunlight. 

 The higher forms of plants contain chlorophyll, and are able to 



FIG. 2. A seedling maple 

 tree showing the pair of seed- 

 leaves (cotyledons) still cling- 

 ing. These have furnished 

 much of the nourishment for 

 the early growth-of the plant. 

 When these drop off the other 

 leaves must supply all the car- 

 bon dioxide from the air and 

 the roots will supply the neces- 

 sary mineral matter from the 

 soil. 



