40 



PRACTICAL BOTANY 



these. As the twigs grow older the green layer is shut away 

 from the light by the corky layer outside of it and soon dies. 

 Most of our useful annuals of the farm and garden, 

 such as corn, potatoes, tomatoes, squashes, and so on, 

 have green stems which do photosynthetic work. 



41. The stem raises leaves into the light. Many 

 plants which cover the ground rather closely do not 



need to raise them- 

 selves much from 

 the earth in secur- 

 ing their share of 

 the light. Good 

 examples of these 

 are such famihar ' 

 creeping plants as 

 white clover, black 

 medic (Medicago), 

 moneywort (Lysi- 

 macJi ia), and some 

 species of wild ev- 

 erlasting (^Ante7i- 

 naria). But very 

 commonly plants 

 compete for Ught 

 against each other, 

 as may readily be 

 seen in almost any 

 cornfield. When 

 the corn is only a 

 little way above 

 ground it is neces- 

 sary to keep it free 

 from rank, quickly 

 growing weeds by 

 frequent cultiva- 

 tion. If this work 



Fig. 25. Branches of Muehlenbeckia, a plant with 

 flattened stems which do most of the photosynthetic 



work of the plant 

 /, flowers ; I, leaves ; s, stems. One half natural size 



