72 PROTECTION FROM INJURIES AND LOSS OF WATER 



OTHER MEANS OF PROTECTION 



Since the epidermis and cork lie at the very exterior they 

 may be classed as preeminently protective tissues; but other 

 tissues lying near the surface may be of a nature to give the 

 cork and epidermis substantial assistance; and although the 

 chief function of these may lie in another direction they can- 

 not 'be passed unmentioned when the function of protection is 

 being discussed. The collenchyma and sclerenchyma tissues 

 belong in this class. 



The origin and general character of the collenchyma has 

 been told on page 30. It forms a somewhat rigid foundation 

 for the epidermis and cork of superficial origin. As stated on 

 page 32, the term sclerenchyma is applied to those cells of what- 

 ever form whose walls are on the whole uniformly thickened 

 and commonly lignified. When short and stocky these are 

 called stone cells, and when long and slender they are termed 

 bast fibers. These not infrequently occur close to or just beneath 

 the epidermis or periderm and help to keep these from caving in 

 and breaking under the stress of a blow or pressure from without. 

 In stone fruits, such as the walnut and peach, it is the stone 

 cells forming the stone that affords the sole means of protection 

 to the seed after the pulp or shell has been removed. 



The borke (p. 55), although a mass of dead tissues in process 

 of elimination, is of no small use as a protective covering; indeed, 

 the very fact that it is dead and dry and hard gives it its protective 

 value. Good illustrations of this are found in the shellbark 

 hickory, sycamore, and grapevine. 



ILLUSTRATIVE STUDIES 



i. Enclose a bit of apple peeling in elder pith (page 251) and 

 cut free-hand sections. Mount some of the thinnest of these 

 in a drop of dilute glycerine. Draw to scale a few of the epi- 

 dermal cells. Measure the cell cavity and the thicknesses of 

 the walls. 



Note the color of the walls in dilute glycerine and mount 



