I20 



BIOLOGY AND HUMAN LIFE 



and underground stems of many plants. There must, therefore, 

 be a current of material passing downward. 



A tree that is girdled (that is, one that has a ring of bark removed) 

 will continue to live for the rest of the season. This shows that the 

 removal of the bark does not interfere with the ascent of water and salts 



from the roots to the leaves. The 

 following spring, however, when the 

 opening of the buds with the rapid ex- 

 pansion of leaves and twigs depends 

 upon food accumulated during the 

 previous summer, the tree will be 

 found dead. Although water and salts 

 may still be able to reach the upper 

 parts of the plant (since the channels 

 that served during the previous sea- 

 son are still open), the food that 

 should have been accumulated during 

 the previous summer is now lacking. 



By far the largest portion of 

 the water that moves from the 

 roots to the leaves evaporates and 

 never comes back. 



The smallest plants that we or- 

 dinarily notice thus show a great 

 division of labor, with special 

 organs and special tissues. 

 98. Our dependence upon chlorophyl. The parts of a plant 

 that have no chlorophyl (for example, the root or the stem of a 

 tree) are unable to make food substances out of inorganic mate- 

 rials and are nourished by materials obtained from the leaves ; 

 but animals and such plants as mushrooms, having no chloro- 

 phyl, must get their food from the bodies of other living things. 

 In the end all food comes from green plants. 



That is to say. through the action of Hght on chlorophyl, the carbon 

 and the oxygen in CO^ become separated so that they are capable of 

 again combining and liberating, or setting free, energy. A carbohydrate 

 may thus serve as a source of energy by becoming oxidized, either in the 



Fig. 72. Bast fibers and vessel 



A, a section cut lengthwise, and B, 

 one cut crosswise, showing bast fibers, 

 sieve-plate vessel, Sp, and the so- 

 called companion cells, cc, found next 

 to the sieve-plate cells. (X 400) 



