230 



TEXTBOOK OF BOTANY 



246. Hairs (Fig. 139). Hairs are usually most numerous 

 on leaves, especially on their under surfaces ; but they are 

 borne also, and sometimes very abundantly, on stems and 

 branches, on various parts of flowers, and on fruits. The 

 downy appearance of the peach is due to a covering of soft, 

 short hairs. Hairs are outgrowths each from a single surface 

 cell, or in some cases from a small group of cells. Often a 

 hair is a single long cell not unlike a root hair. One-celled 

 hairs are sometimes bent so as to lie nearly parallel with the 



surface of the leaf 

 that bears them ; 

 in such a case, if 

 they are numer- 

 ous, they give a 

 silky appearance 

 to the leaf. In 

 other cases the 



ge- 



A 



FIG. 139. Hairs: A, of the common 

 ranium (Pelargonium) ; B, of Correa speciosa; 

 C, of wormwood; D, of mullein. B, C, and D 

 after Kerner. 



hair is composed 

 of a row of cells, 

 or even of several 

 rows side by side. 

 Sometimes it is 

 branched, like the 

 hairs of the mullein ; sometimes instead of a hair a plate 

 of cells is formed, like the chaffy scales of many ferns. In 

 general, hairs are useful to plants because they prevent the 

 too rapid evaporation of water. 



Many plants living in very dry places, which can ill afford 

 'to give off much water to the air, have especially hairy leaves 

 and stems. Thus the branched hairs of the mullein form a 

 woolly coating which largely prevents currents of air from 

 reaching the. surface of the leaves and stem, and so prevents 

 the rapid carrying away of moisture. The woolly coatings 

 of some winter buds are useful in the same way, because 

 during winter the roots cannot supply any considerable 



