192 BOTANY AND PHARMACOGNOSY. 



Protecting cells include those cells which are located on the 

 outer parts of the plant. The function of these cells is to lessen 

 the rate of transpiration, or the giving ofif of water; to furnish 

 protection against changes of temperature, and to protect the 

 inner tissues against the attack of insects ; they also have a me- 

 chanical function (Figs. io6; iii, E). 



Depending principally upon their composition, these cells may 

 be tUvided into two classes, namely, epidermal cells and cork cells. 



The epidermal cells constitute the outermost layer of the 

 plant. They contain cytoplasm but the plastids in some instances 

 are wanting ; in some instances they also contain dissolved color- 

 ing principles ; and on account of the relatively large amount of 

 water which they contain, they are classed among the important 

 water-reservoirs of the plant. 



The outer walls are principally characterized by one or more 

 lamellae of cutin, these uniting to form a continuous wall. The 

 cutin is often associated with wax, this constituting the bloom of 

 fruits ; less frequently such inorganic substances as calcium car- 

 bonate, calcium oxalate an^ siiica are present, and not infrequently 

 mucilage is present, as in the walls of certain seeds ( Fig. 99, A ) . 



On surface view the form of these cells varies from nearly 

 isodiametric to oblong ; they may also be polygonal or branched. 

 In transverse section their radial diameter is much the shorter. 

 In some instances the inner and side walls are considerably thick- 

 ened, as in the seeds of a number of the Solanacese (Fig. 302, A). 



The epidermis usually consists of a single layer of cells, but 

 may have additional layers underneath forming the hypodermis, 

 as in the upper surface of the leaves of species of Ficus ; in some 

 instances the hypodermis undergoes a mucilage modification, as 

 in the leaves of buchu. (See also Figs. 99, A; 100.) 



Plant Hairs. The epidermal cells are sometimes specially 

 modified centrifugally, giving rise to papillse, to which the velvety 

 appearance of the petals of flowers is due ; in other cases this 

 modification is in the fomi of hairs or trichomes (Figs, no, 

 118, 283, 284). These may be imicellular or multicellular, and in 

 addition the latter may be glandular or non-glandular. Glandular 

 hairs possess a head-like apex, consisting of one or more cells, 

 and they secrete oil. mucilage and other substances (Fig. 285). 



