68 PROTECTION FROM INJURIES AND LOSS OF WATER 



on slight pressure and the jagged edges of the fracture prick 

 through the skin, while a stinging fluid is injected into the wound 

 by the pressure that breaks off the point, and doubtless also by 

 the elasticity of the turgid cell. In Humulus Lupulus, Phaseolus 

 multiflorus, and Loasa hispida we find excellent examples of 

 hairs that act as hooks to assist plants in climbing (Fig. 33), 

 while in the branched and interlaced hairs of Verbascum Thapsus 

 and the stellate scales of species of Abutilon, Olea and Croton 

 are efficient devices for reducing transpiration and reflecting 

 a part of the sun's rays. The reduction of transpiration is 

 brought about largely by the formation of dead air spaces 

 between the interlaced hairs and beneath the scales. 



Hairs that are used for absorption and the secretion of special 

 materials will be discussed in the chapters on absorption and 

 secretion. 



The Multiple Epidermis. — Sometimes the outer layer of 

 epidermal cells is underlaid by one or more layers similar to it 

 in content. These cells may have arisen from a division of 



the protoderm, in which case they 

 -^QoOoQ^^^^ are morphologically or by descent 



^j- — yr — ^nf'^^ ir~lr epidermal, or they may have 



^^'^i^^^xrSriAir sprung from the ground meristem 

 ^^^==:d-^;^:=====^^^^^^ beneath the protoderm, and then 



Fio. 34.-Multiple epidermis of they WOuld bc not StHctly Cpi- 

 leaf of mangrove in cross section. dermal, although SO claSSCd by 

 This serves as a water reservoir, . 



and the relatively thick walls of the general usagc. Whatever their 



inner cells reenforce the protective qj-J^J^, thcSC Cclls, together with 



power of the outer layer. Ihe o ' ' ^ 



mangrove grows in the salt soil of sea- thoSC of the OUtCr layer, are Col- 



lectively called the multiple epi- 

 dermis (Fig. 13). The inner or accessory cell layers, as a rule, 

 seem to serve almost exclusively as water reservoirs. In most 

 cases their walls remain cellulose and thin and their cavities 

 become relatively large. As in the outer layer, the protoplast is 

 a thin film lining the walls, and the cell cavity is filled with a 

 clear fluid, of course mostly water. Since these accessory layers 

 are chiefly for holding water they should be discussed in the 



