72 Elementary Botany 



Besides stomata, the epidermis is very frequently provided 

 with hairs of various kinds. 



Tliese are simply prolongations of the epidermal cells, 

 sometimes unicellular (fig. io6, i.), sometimes multicellular 

 (fig. io6, II.). 



They vary much in their shape, sometimes being simple 

 and at other times branched. 



An interesting modification is seen in glandular hairs which 

 contain various secretions ; of these stinging hairs, as are found 

 in the stinging-nettle, are good examples. In this case there 

 is a little bag at the base of the hollow hair, containing an 

 acrid fluid, and surrounded by a number of elastic cells. The 

 point of the hair is sharp, and is protected by a little cap. 

 Wnen the nettle is touched lightly, the cap is broken off, the 

 point of the hair pierces the skin, and a drop of the stinging 

 juice is forced into the blood. If the nettle be grasped firmly 

 the sting is broken lower down, and hence there is no sharp 

 point to pierce the skin, and the juice is simply poured out upon 

 the exterior. 



2. The surface of the epidermis is generally covered with a 

 thin structureless layer, the cuticle (fig. 105, a and/). 



3. The parenchyma of the interior, of the leaf consists of 

 cells containing chlorophyll. 



There is a difference, however, in their arrangement in the 

 different parts of the leaf. Towards the upper surface the cells 

 are packed closely together (c, fig. 105), whilst towards the 

 under surface we find examples of stellate parenchyma, leaving 

 air spaces between the cells {d, fig. 105). This is the cause of 

 the fact that in most leaves the under surface is lighter in colour 

 than the upper, as there is less chlorophyll packed there. 



The upper parts of the veins are in direct communication 

 with the wood of the stem, whilst the lower parts are connected 

 with the liber. At the edges of the leaves there is a passage 

 between the upper and lower sides, so that the sap which 

 passes up the wood flows out through the upper part of the 

 veins, back through the under surface to the liber, and so down 

 the stem. 



There is a great difference in the venation or arrangement 



