PLANT ORGANS AND ORGANISMS 155 



THE LEAF 



The leaf is a usually flattened, rarely semi-centric, or centric-lateral 

 expanse developed by the stem or by branches and in whose axil one 

 or more branches arise. 



Leaves seldom develop buds over their surface or along their 

 margin and in connection therewith roots. The capacity for bud 

 development is restricted to three families, viz.: Crassulacece, Be- 

 goniacea and Gesneracea. 



Leaf Functions . The most essential function of plants is the con- 

 version of inorganic into organic matter; this takes place ordinarily 

 in the green parts, containing chlorophyll, and in these when exposed 

 to sunlight. Foliage is an adaptation for increasing the extent of 

 green surface. 



The functions of a leaf are photosynthesis, assimilation, respiration 

 and transpiration. 



Photosynthesis is the process possessed by all green leaves or other 

 green parts of plants of building up sugar, starch or other complex 

 organic substances by means of chlorophyll and sunlight. This 

 process takes place in nature, only during sunlight. CO 2 is taken 

 in and O given off. 



Assimilation is the process of converting food material into proto- 

 plasm. 



Respiration or breathing is the gaseous interchange whereby all 

 living organisms take in oxygen and give off carbon dioxide. 



Transpiration is the giving off of watery vapor. 



Types of Leaves Developed in Angiosperms. These may be 

 tabulated as follows: 



1 . Cotyledons (the primitive or seed leaves) . 



2 . Scale leaves. 



3. Foliage leaves. 



4. Bract leaves: (a) primary at base of inflorescence: (b) bracteo- 

 lar leaves at a base of individual flowers. 



5. Sepals. 



6. Petals. 



7. Microsporophylls (stamens). 



8. Megasporophylls (carpels). 



