MORPHOLOGY OF HIGHER PLANTS. 



195 



Cork not only occurs as a secondary protective layer, but may 

 also arise in other parts of the plant as a result of injury, as in 

 leaves, fruits, stems and tubers. It also arises as a result of the 

 disarticulation of the leaf in autumn. 



Lenticels may be described as biconvex fissures in the cork 

 which permit of the easy access of air to the intercellular 

 spaces of the rather loosely arranged cells lying beneath them 

 (Fig. 107). They usually arise as the product of a meristem 

 situated beneath the stomata of the epidermis, the stomata being 

 replaced by them when cork is developed. Several types of lenti- 



FlG. 107. Section through a secondary lenticel in the bark of Sassafras; .e, epidermis, 

 st, stone cells; phel, phelloderm derived from secondary phellogen and having thick ligni- 

 fied wall; p, parenchyma; c, cork; com, complementary cells. After Weiss. 



eels are distinguished. They are quite characteristic and promi- 

 nent in a number of barks, as those of species of Betula, Prunus, 

 Rhamnus (Fig. 229), etc. 



Laticiferous or milk tissue occurs in all those plants which 

 emit a milk-juice on being cut or otherwise wounded. The juice 

 may be colorless, as in the oleander ; whitish, as in the Asclepia- 

 dacese and Apocynaceje (Fig. 202) ; or yellowish, as in the Papa- 

 veracege. It contains caoutchouc, oils, resins, mucilage, starch, 

 calcium oxalate and alkaloids as well. The walls are relatively 

 thin and consist chiefly of cellulose. The tissue consists either 

 of single cells of indefinite length, as in the Asclepiadaceae, or it 



