MORPHOLOGY OF LEAVES AND FLOWERS 



603 



for the escaping moisture, the leaf is able to check transpiration. 

 The various epidermal modifications which are quite common among 

 the weeds afford protection. In some cases this consists of a waxy 

 layer on the outside of the epidermis as in some milkweeds and some 

 species of wild lettuce. This layer of wax prevents the escape of 

 moisture, and protects the chlorophyll-bearing tissue of the leaf 

 from the intense light rays. 



Another very common protective structure upon the leaves is 

 to be found in the great variety of hairs developed by the epidermis. 

 In the mullein the hairs are so prominent that they form a felt- 

 like covering. Among the cinquefoils and thistles, the hairs are 

 usually not branched as in the mullein and the covering is not so 

 dense. 



Fig. 456. Urticating hairs and cutting leaves, a, urticating hair of Nettle; 

 b, bristle of Bugloss ; c, barbed margin of a leaf of Sedge; d,. barbed margin 

 of a leaf of Grass. 



All stages from those in which the hairs are very small, giving 

 the leaf a downy appearance, up to the extreme case in the mullein, 

 can be found among the weeds. 



