PLANT HAIES 



29 



Periderm. — -Periderm is a name applied to all the tissue, produced 

 externally by the cork cambium (phellogen). This term appears 

 often in pharmacognic and materia medica texts. 



Histology of Typical Monocotyl Stems (Endogenous). — Passing 

 from exterior toward centre the following structures are seen: 



1 . Epidermis whose cells are cutinized in their outer walls. 



2. Hypodermis, generally 

 collenchymatic. 



3. Cortex. 



4. Endodermis or innermost 

 layer of cortex generally with 

 greatly suberized cell walls. 



5. A large central zone of 

 parenchyme matrix in which are 

 found scattered fibrovascular 

 bundles of the closed collateral 

 or rarely concentric t3rpe (am- 

 phivasal). In this latter type, 

 which is typical of old mono- 

 cotyl stems, the xylem grows 

 completely around phloem so 

 that phloem is found in the 

 centre and xylem without and 

 surrounding it. 



Plant Hairs or Trichomes 



Fig. 23. — Cinchona calisaya. Cross-sec- 

 tion of bark. A, Cork cells; B, cortical 

 parenchyma; C, stone cells; D, phloem por- 

 tion; E, soft bast; F, phellogen forming 

 bark; G. medullary rays. (The black line 

 from G should be extended to the parenchyma 

 cells between the phloem portions.) H, Bast 

 fibers. {From Sayre.) 



These are out-growths of 

 the epidermal cells which have 

 become greatly elongated and 

 may be unicellular or multicellular. They may be of various forms : 

 simple, consisting of a single row of cells; branching; clavate, or club- 

 shaped; stellate or star-shaped; barbed, hooked, forked, etc. 



The terminal cell is often modified into a secretion sac for gummy, 

 resinous or odorous products. Such hairs are called glandular. Ex. : 

 Glandular hairs from strobiles of Humulus lupulus. 



The cotton of commerce which is the hairs of the seed of the 

 cotton plant, Gossypium herbaceum, is a good example of simple 

 hairs. 



