STUDIES OF SEED-PLANTS 179 



because proper cultivation of corn has left the soil in con- 

 dition for planting wheat without replowing, and also because 

 in the corn-growing regions oats and corn must be planted in 

 the spring and there would be little time for planting spring 

 wheat and rye. Still another example : many of our garden 

 biennials (carrots, beets, cabbage, onion, turnips) will bloom 

 and form seed in one long season in a warm climate, or some- 

 times if forced with rich soil and plenty of water. The above 

 are simply examples taken from a long list of cultivated plants 

 whose duration of life man has been able to change. 



Home-work : Make lists of the common plants you know, classify- 

 ing them as annuals, biennials, or perennials. 



169. General Functions of Stems. The primary work of 

 ordinary stems is (1) support of the leaves in positions adapted 

 to their work of food-making and transpiration, (2) conduc- 

 tion of materials between roots and leaves, and (3) breathing. 



(1) In its work of support stems make use of the elasticity 

 and rigidity of woody tissues and also of turgidity due to 

 water in the cells. A very young plant depends largely upon 

 turgidity, for if water is withheld from the soil, the leaves and 

 stems become wilted. A young Windsor-bean plant growing 

 in a pot will illustrate this. If the soil be allowed to dry, the 

 stem will lose its turgidity (become wilted) and will gradually 

 bend over until the leaves touch the soil. If then water is 

 added to the soil, the stem will become turgid and rigid with 

 remarkable rapidity, and within two or three hours will be as 

 erect as ever. Stems of older plants of most species do not 

 lose their rigidity when water is wanting, for woody tissues 

 furnish the necessary rigidity. A straw of wheat or rye is a 

 splendid example of rigidity due to woody tissues arranged 

 in cylindrical form. 



(2) The work of conducting materials is, as we have seen 

 in the bean plant, due largely to the tubes in wood and bark 

 which are specially fitted to conducting liquids lip and down 



