284 TISSUE ELEMENTS OF SEED PLANTS 



yellow). Lignified walls also take up the aniline dyes 

 strongly, so that the walls of the tracheids and vessels 

 and often also of the fibres are deeply coloured in 

 permanent microscopic preparations of plant tissues 

 that have been stained with these dyes. Lignification 

 hardens the wall considerably, but does not decrease 

 its permeability to water. 



In contrast to lignification, in which lignin is laid 

 down in a cellulose matrix, is the formation of parts 

 of certain cell walls, not by cellulose, but by one of 

 two aggregate substances known as cutin and suberin. 

 These are substances which differ chemically from one 

 another, but are closely allied and have the same 

 physical property of being impermeable to water. 

 They are not true fats, but they are allied to fats and 

 show some of the same reactions, for instance staining 

 with Sudan 3. Cutin is the substance of the cuticle 

 covering the epidermis of the shoot, and suberin forms 

 one of the layers of the wall in cork cells. Hence 

 both the outer walls of the epidermis of the herbaceous 

 shoot and also cork tissue (which is formed in the bark 

 of woody plants, see Chapter XX) are practically 

 impermeable to water, and thus prevent the drying 

 up of the shoot by evaporation. The impermeability 

 of cork to water accounts of course for the use of the 

 bark of the cork oak (Quercus suber), which is very 

 pure cork, to make bottle corks. 



The contrast between cutin and cellulose can be 

 demonstrated by treating a section through the epidermis 

 with Schulze's solution (solution of iodine in zinc 

 chloride) which stains the cellulose blue, owing to the 

 formation of " amyloid/' a substance that gives, like 

 starch, a blue colour with iodine, but the cutin of the 

 cuticle yellow. 



