Stems and Their External Features 105 



Conductive tissues necessary in stems. The simplest land 

 plants are very small and grow flat on the soil in wet places. 

 They are constantly in contact with the moist soil, and their 

 cells can be supplied almost directly with water and mineral 

 salts. In such plants a conductive system is not necessary ; 

 but if the leaves of a plant are to be raised into the air, water 

 for transpiration must not only be supplied to them con- 

 tinuously, but at times it must be supplied in great quantity. 

 Because of this fact, a plant that raises its leaves even a few 

 inches above the soil must possess conductive tissues, and 

 when large numbers of leaves are raised 200 or 300 feet into 

 the air, a very extensive water-conducting system is neces- 

 sary. The water-conducting tissues are of vital importance in 

 the stems of complex plants. 



The roots and stems require a continuous supply of food for 

 repairing old cells and for building new ones. Since the foods 

 are manufactured primarily in the leaves, there must be food- 

 conducting tissues that are adequate to carry them to all 

 parts of the stem and roots. The food-conducting tissues 

 also transfer food from the leaves to the seeds and growing 

 parts, and when food has accumulated in the stem or roots 

 it may pass up through the conductive tissues of the stem to 

 other parts of the plant. Food-conducting tissues are neces- 

 sary in stems to transfer food within the plants. 



The stem as a place of food accumulation. Because of the 

 volume of the stem it is natural that excess food should ac- 

 cumulate in it. Under favorable conditions, foods for which 

 the plant has no immediate need are continuously passing 

 from the leaves into the stem. Consequently, in our larger 

 plants the stem is the place of temporary storage and the center 

 from which foods are distributed as needed. In some plants 



