Physiological Functions of Plant Tissues. 263 



liave very often a wavy irregular outline. This may 

 perhaps be due to their affording in this way a longer 

 surface of attachment. One also sees abundant chlorophyll 

 in the guard cells of the stomata, but in the other epidermal 

 cells of the upper leaf-surface it is sparingly present. 



Now, the palisade parenchyma lies directly under the 

 epidermis, hence much chlorophyll in the cells above would 

 interfere with the illumination of the palisade parenchyma. 

 Perhaps tlie convexity upwards of the epidermis cells may 

 also enable them to act as lenses collecting the light on to 

 the underlying palisade parenchyma. The thick coating 

 of hairs on many desert and steppe plants may also prevent 

 transpiration. (It is interesting to compare the leaf of 

 Populus alba, which is freely movable in the wind, and has 

 a thick covering of hairs on the under surface, with that of 

 P. nigra, which is fixed, and has no hairs.) 



(6) Periderm, or the Phellogen and its products. 



Cork. — The chief characteristic of the cork cells lies in 

 the suberin layer, which is not penetrated by the pore canals. 

 This suberin is extremely impermeable for water, gases, and. 

 heat. It is also very flexible and elastic. One of its 

 functions is to heal wounds, and the bitter tannin and other 

 principles found in it may help in protecting the plant 

 against insects. 



Phellogen. — This consists of thin- walled meristem cells 

 full of protoplasm, which divide tangentially, the outer layer 

 usually becoming cork, while the inner remains as phellogen. 

 Sometimes it also forms chlorophyll parenchyma or 

 phelloderrn. At first the phellogen is immediately beneath 

 the epidermis. Later, however, it is deeper in position, and 

 the superficial parts thus cut off from it get dried up and 

 form the periderm. This periderm can only be of use as a 

 protecting layer, and the old and functionally useless rind 

 parenchyma subserves the same purpose. It is sometimes 

 thrown off by specially developed separation layers derived 

 from the cork tissue, but without suberin. 



(c) Development of the Skin System. — Epidermis is usually 

 derived from dermatogen, but in the perforated leaves of 

 Arum and Philodendron a perfectly developed epidermis is 

 derived from the ground parenchyma round the holes. The 

 phellogen never appears directly at the growing point. 



