LEAVES AND THEIR FUNCTIONS 



137 



~^P^ 



Into one end of a glass tube fit a rubber stopper with a hole through it 

 Into this hole insert the stem (freshly cut off under water) of a leafy branch 

 from an actively growing tree. Seal the stem* in with melted wax. Fill 

 the tube with cool, boiled water, and immerse the 

 open end in mercury as shown in the figure. 



Respiration by Leaves. — All living things, 



with the possible exception of some bacteria, 



require oxygen in order to live. It is by 



means of the oxidation of food materials 



within the plant's body that the energy used 



in growth and movement is released. A 



plant takes in oxygen largely through the 



stomata of the leaves, to a less extent 



through the lenticels in the stem, and through 



the roots. In young plants, especially, much 



oxygen is taken up by the latter-named 



organs. Thus the rapidly growing tissues 



receive the oxygen necessary for them to 



perform their work. It can be shown by 



experiment that a plant uses up oxygen 



in the darkness ; in the light the amount of 



oxygen given off as a by-product in the process of starch making 



is, of course, much greater than the amount used by the plant. 

 Summary. — From the above paragraphs it is seen that a 



leaf performs the following functions: (1) breathing, (2) starch 



making, with the inci- 

 dental passing out of 

 oxygen, (3) formation of 

 proteids, with their diges- 

 tion and assimilation to 

 form new tissues, and 

 (4) the transpiration of 

 water. 



Modified Leaves.'^ Leaves 

 as Spines. — Examine a holly 

 leaf. Of what use might the 



Apparatus to show the up- 

 ward pull of leaves. 

 (After Detmer.) 



Modified leaves (holly). Note the spines. 



stiffened spines be to the plant? To what dangers might such a leaf be ex- 

 posed? Remember that holly keeps green in a much colder atmosphere 



1 For exercises on modified leases, see Hunter and Valentine, Manual, pages 

 40, 41, 42, 45, 46. 



