470 A TEXTBOOK OF THEORETICAL BOTANY 



Papillate surfaces are usually velvety in texture. If hairs are scanty I| 



the surface is called hispid, while if they are numerous they form a 

 woolly or tomentose surface. If they are absent the surface is called 

 glabrous. 



The more advanced types of hairs are multicellular and, like the unicellular 

 types, they may be simple or branched. The branching is often complex 

 and gives rise to a multitude of varied forms. The branches sometimes 

 become closely interlaced so that the epidermal surface is quite hidden from 

 view. Dense coverings of hairs are traditionally reputed to protect leaves 

 against excessive evaporation, but experiment does not altogether support 

 this idea. For root hairs see Chapter XX. 



Hairs may also be formed by the combined outgrowth of a group of 

 epidermal cells. Such hairs commonly form secretory glands (see below). 



Fig. 458. — Rosa sp. showing stem prickles. 



Sometimes, however, they become woody and comparatively massive, and 

 are then known as prickles (Fig. 458). The Rose and the Blackberry provide 

 good examples. Unlike true thorns (see Chapter XXI) they are purely 

 epidermal structures and usually contain no vascular tissues (Fig. 459). 

 Moreover, they may occur anywhere on the plant, on stems, leaves, or even, 

 rarely, on roots or flowers. Hairs may also be flattened into scales, closely 

 covering exposed surfaces. 



Glands 



Glands are classified physiologically as either secretory or excretory, 

 but it is difiicult to draw a clear line between the two functions. Broadly 

 speaking a secretion is directly valuable to the plant and an excretion is 

 a superfluous or deleterious substance. Plants have no regular method 

 of evacuating their excretions as animals have. They dispose of them 

 internally, building them into their structure, usually in superannuated 



