CHEMICAL REACTIONS OF PLANT TISSUES 59 



Cutinized and Suberized. Walls. — The substance 

 cutin occurs in the epidermal tissues, especially in 

 those of the green parts of the plant. These cutinized 

 walls form a thin, transparent, waterproof membrane 

 or covering tissue, which protects delicate structures 

 from injury and excessive evaporation of water. As 

 the plant matures the epidermal tissues are gradu- 

 ally replaced by cork, which contains the substance 

 suberin in the walls of its cells. Cork or suberized 

 cells protect the mature plant against temperature 

 changes and mechanical injury. Cutinized and sub- 

 erized cell walls give similar reactions with most 

 reagents and for differentiation we must depend upon 

 structure and location rather than upon chemical 

 properties. The following reactions are chiefly of 

 use in distinguishing between cellulose or lignin and 

 cutin or suberin: 



1. Concentrated potassium hydroxide solution (30 per cent) 



produces a yellow to brown coloration in suberized walls. 



2. Concentrated alcoholic chlorophyl solution, acting in the 



dark, stains cutinized and suberized walls green, but does 

 not affect cellulose or lignified walls. 



3. Alkannin solution colors cutin and suberin red. 



Gums and Pectinous Substances. — The cell wall 

 in certain plants is so modified in its chemical nature 

 that the original cellulose has been more or less replaced 

 by substances which, in contact with water, behave 

 like the gums and form mucilages. It may be that 

 the gummosis occurring around wounds in certain plants 

 is an effort to prevent injurious substances from 

 reaching vital parts. However, gums are occasionally 

 found in seeds apparently well protected from injury 

 by tough coats. As the gums are complex chemical 



