GERMINATION OF SEEDS 3 



stems. They retain such parts of this as they may require. In 

 daylight they retain chiefly the carbon dioxide and a part of this is 

 usted to make starch and sugar in the cells of the leaves. The sur- 

 plus oxygen is given off to the air again. (Exercise I.) At night 

 the plant uses more oxygen than carbon dioxide. It then breathes 

 off the latter into the air. 



Light Requirements. — As just stated, the presence of light is 

 necessary for the making of starch and sugar in plant cells. The 

 green color bodies called chlorophyll, are also necessary. A green, 

 plant, growing in the sunlight, is truly a starch and sugar factory. 

 The products of this factory are used by the plant to build up the 

 tissues, and this is called growth. We see then, how essential sun- 

 light is to plant growth. 



Leaf Function and Structure. — The leaves may be considered 

 as both the breathing organs and digestive organs of plants. There 

 are numerous openings, or, mouths, called stomates, more numerous 

 on the under surfaces of leaves. Through these the air contain- 

 ing oxygen and carbon dioxide is admitted and the by-products are 

 given off. In the daylight the chief waste is oxygen, because "food 

 getting" exceeds the "breathing." In the darkness the chief 

 waste is carbon dioxide, because "breathing" exceeds "food 

 getting." 



The soft cells inside the leaf are provided with many small 

 bodies bearing green chlorophyll. This gives the green color to the 

 leaf. The upper and lower layers of epidermis are composed of 

 transparent cells through which the light passes readily. The layer 

 of cells just beneath the upper epidermis are arranged in a columnar 

 way as shown in the figure, and are called palisade cells. The 

 others are more irregular and have spaces between them for the 

 access of air which bathes the cells. All the thin-walled cells bear- 

 ing chlorophyll, inside the leaves, are called parenchyma cells. 

 The essential parts of a typical leaf are shown in figure 1. 



Freedom from Enemies. — Among the conditions necessary for 

 plant growth is freedom from insect enemies, plant diseases, and 

 weeds. For the best growth of garden and fruit crops, enemies of 

 all kinds must be kept in check. Each of these classes of enemies 

 is discussed in other chapters. 



Germination of Seeds. — The first growth from seeds is called 

 germination. The term ' ' sprouting' ' more properly refers to break- 

 ing through the ground by this early growth. The seed contains 

 the minute parts of the young plant. There is a store of plant food 



