FIELDBOOK OF ILLINOIS WILD FLOWERS 



milkweed, respectively. Common chickweed is an example of 

 a plant bearing ol;«/ leaves, f. 



CHARACTERISTICS OF LEAF I'ARTS 



Tips, bases and margins of leaves are described in a number 

 of ways that assist and sometimes clinch the identification o\ 

 plants. Bases and tips may commonly be referred to as rounded 

 or narrowed. Short blunt tips are called obtuse^ sharp acute, 

 and long sharp points acuminate. The margins of leaves or 

 leaflets may be, as shown also in fig. 2, lobed, a, toothed, b, or 

 entire, c. Teeth may be coarse or fine, sharp or blunt; or the 

 leaf may be doubly toothed — having large teeth whose margins 

 have small teeth, fig. 2 c. The lobes or the teeth vary greatly. 

 They may be small or large, round or pointed, deep or shallow, 

 and their margins may be entire or toothed. The indentation 

 between two lobes is called a sinus. 



Leaves originate as outgrowths from the sides of developing 

 young stems while these are still in the bud. At the nodes or 

 points of attachment for the leaves there may frequently be seen 

 buds just above the leaf or above the scar left by the leaf when 

 it drops. This position in the angle between leaf and stem is 

 called the axil of the leaf, and buds developing there are axillary 

 buds. 



ARRANGEMENTS OF LEAVES ON STEMS 



There are three possible arrangements for the leaves at 

 the nodes. If there is one leaf at a node the arrangement is 

 spiral, sometimes called alternate, since the leaves range round 

 the stem so that no one is directly over the leaf at the node 

 next below. Two leaves at a node form the arrangement called 

 opposite; and three or more at a node the whorled. A whorl 

 of leaves immediately below a flower or flower group is called 

 an involucre; a secondary whorl of modified leaf structures 

 above the involucre is called an involucel. 



Some or all of the leaves may be basal. In such case they 

 spring from or near the base of the stem, and may be few and 

 sessile or on long petioles, or many forming a rosette at the surface 

 of the soil. 



Leaves divide into two great classes according to the length 

 of time they remain on the stem. If they stay all winter, per- 

 sisting until after new leaves develop, they are evergreen; how- 

 ever, it they drop off in autumn they are known as deciduous. 



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