PLANT LIFE. 27 



carbon dioxide, of which a small amount is always 

 present in the air and in water. 



Plants Require Power. In the plant when its cells 

 begin to manufacture food out of the raw materials, 

 water and carbon dioxide, we look for the source of 

 the power and find it in the sunlight. Keep a plant 

 in the dark and it will grow as long as the stored-up 

 food lasts, but then dies, because it cannot manufac- 

 ture any food in the absence of light. Ordinarily a 

 plant manufactures enough food by day to last it over 

 night, with some to spare. Trees that shed their 

 leaves produce enough food during the summer to 

 keep them alive all winter, and furnish the material 

 out of which all the new leaves and twigs are made, 

 until the new leaves are able to manufacture food for 

 themselves. 



Plants Require Light. Thus it is clear why many 

 plants do poorly in the shade; they do not get enough 

 light to enable them to keep up their food supply. 

 Trees lighted only on one side grow out in that direc- 

 tion, while the branches on the shaded side remain 

 small, hence the trees become unsymmetrical. The 

 so-called " smothering out " of some weeds, by a rap- 

 idly growing crop, is in part due to shading them so 

 that they get insufficient light, and are so weakened 

 that they cannot secure all the water and mineral mat- 

 ter they need from the soil. 



Plant Requirements. A green plant, then, if it has 

 plenty of light, oxygen, and a sufficient supply of 

 water and carbon dioxide, has no need of any food 

 from the outside. 



Plants Require Mineral Substances. In addition to 

 these, however, a plant needs certain other substances. 

 There are certain mineral substances needed to take 

 part in the building of new protoplasm and new cell 

 walls, and for certain other purposes. These min- 

 erals are found in the soil, in solution, in the soil 

 water, and are absorbed with the latter by the plant. 

 Most of these necessary mineral substances are present 



