42 



readily be found, since leaves taken from the same tree or branch 

 often show a wide variation. 



The margins of leaves are often more variable than their apexes 

 and bases. The kinds most commonly recognized are shown on 

 Plate V. The figures represent the margins' of simple leaves, but 

 the margins of the leaflets of compound leaves follow the same ter- 

 minology. 



Most of the leaves of our common forest trees contain a rather 

 complicated system of fib ro- vascular bundles. These fibro-vascular 

 bundles, known as veins, form the framework of the leaves. Sur- 

 rounding and between these veins is found a green pulpy mass, the 

 spongy parenchyma. The whole body of the leaf is covered by a 

 protective covering known as the epidermis, the thickness of which 

 varies with the species of tree and the climate. 



One can find variations in the petiole and stipules of leaves 

 as well as in the blade. The petiole may be absent, short, or long. 

 When the petiole is absent the leaf-blade is sessile. It may also be 

 enlarged at the base, circular, heart-shaped, flat, or triangular in 

 outline. The enlarged base may be hollow or clasping. The stipules 

 are usually not very conspicuous. In many species they persist for 

 a short time only and then fall off. The main function of the stipules 

 is protection, but a special modification of the stipules is seen in the 

 Common Locust (Plate XCVII), where the thorns are modified stip- 

 ules and function as mechanical protectors. 



Leaves are the most industrious organs of a plant. They work 

 day and night from early spring until autumn. The four chief func- 

 tions of leaves are: (1) Photosynthesis; (2) Kespiration; (3) 

 Transpiration, and (4) Assimilation. Photosynthesis is the process 

 by which the leaf manufactures starch or sugar from carbon dioxide 

 and water with the aid of the energy of light. That green plants 

 require light for their growth and development is shown by the man- 

 ner in which the axis and their leaves adjust themselves so as to re- 

 ceive the greatest amount of light. By respiration in plants is meant 

 the process by which oxygen is consumed and carbon dioxide and 

 water are given off. It is primarily a process of oxidation and re- 

 sembles in general the process of respiration as found in man and 

 higher animals. In order to facilitate this exchange of gases the 

 plants are supplied with openings on the leaf surfaces, especially on 

 the lower surface, and on the bark. The openings on the leaf sur- 

 faces are known as stomata and those on the bark as lenticels. Each 

 slit-like opening on the leaf is surrounded by two guard cells which 

 are somewhat complicated in structure and very sensitive to changes 

 in temperature and water supply. They function primarily as 

 breathing pores and as outlets for the water vapor given off during 

 the process of transpiration. Their number varies, but it has been 



