ORGANIZATION: PLANT TISSUES. 



359 



in plants growing in dry situations or when; they are exposed to drying 

 conditions. The epidermal cells generally become considerably flattened, 

 and are usually covered with a more or less well developed water-proof 

 cuticle, a continuous layer over the epidermis. In many plants the cuticle 

 is covered with a waxy exudation in the form of a thin layer, or of rounded 

 grains, or slender rods, or grains and needles in several layers. These 

 waxy coverings are sometimes spoken of as "bloom" on leaves and fruit. 



707. Trichomes. Trichome is a general term including various hair- 

 like outgrowths from the epidermis, as well as scales, prickles, etc. These 

 include root hairs, rhizoids, simple or branched hairs, glandular hairs, 

 glandular scales, etc. Glandular hairs are found on many plants, as 

 tomato, verbena, primula, etc.; glandular scales on the hop; simple-celled 

 hairs on the evening primrose, cabbage, etc.; many -celled hairs on the 

 primrose, pumpkin; branched hairs on the shepherd's purse, mullein, etc., 

 stellate hairs on some oak leaves. 



For stomates see Chapter IV. 



4. OAIGIN OF THE TISSUES. 



708. Meristem tissue. The various tissues consisting of cells of dissimi- 

 lar form are derived from young growing tissue known as meristem. Meri- 

 stem tissue consists of cells nearly alike in form, with thin cell walls and 

 rich in protoplasm. It is situated at the growing regions of the plants. 

 In the higher plants these re- 



gions in general are three in 

 number, the stem and root 

 apex, and the cambium cyl- 

 inder beneath the cortex. 

 Tissues produced from the 

 stem and root apex are called 

 primary, those from the cam- 

 bium secondary. In most 

 cases the main bulk of the 

 plant is secondary tissue, 

 while in the corn plant it is all 

 primary. 



709. Origin of stem tissues. Section through owg point of stem, d, 

 lust back of the aoical dermatogen; p, plerome; periblem between. 



, (After De Bary.) 

 menstem in a longitudinal 



section of a growing point it can be seen that the cells are undergoing a 

 change in form, and here are organized three formative regions. The 

 outer layer of cells is called dermatogen (skin producer), because later it 

 becomes the epidermis. The central group of elongating cells is the plerome 

 (to fill). This later develops the central cylinder, or stele, as it is called 



