PLANT ORGANS AND ORGANISMS 
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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 - 
goniacece and Gesneracece. 
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. C 0 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). 
