CHEMICAL CONSTITUENTS OF CELL WALLS 69 



and several workers have suggested that its molecular weight lies between 800 

 and 900. Brauns and Hibbert (1933) have proposed the empirical formula 

 C47H52O1G to represent the structural unit of spruce lignin but no empirical 

 formula has received general acceptance. 



Lignin first appears in the middle lamella and primary wall and later can 

 be detected in the secondary wall. When associated with cellulose, lignin is 

 present in the spaces between the micelles and not within the micelles. Ligni- 

 fied walls are usually freely permeable to water and solutes. The tensile 

 strength of lignified cell walls is the same as that of cellulose walls but 

 lignified walls resist compression better than cellulose walls. The increased 

 resistance of lignified walls to compression is explained by the assumption that 

 the presence of lignin in the intermicellar spaces welds the cellulose micelles 

 into a single coherent mass and thus prevents the bending and buckling of the 

 cellulose strands when they are subjected to compression strains. 



Ciitin is the name applied to the mixture of wax-like materials found on 

 the outer surface of the epidermal cell walls of leaves, stems, fruits, and other 

 organs. These wax-like substances are intimately associated with cellulose 

 and often with pectic substances producing a wall of great structural com- 

 plexity (Meyer, 1938). Cutinized cell walls are relatively impermeable to 

 water. The presence of cutin in the outer walls of epidermal cells greatly 

 reduces the evaporation of water from the surfaces of plant tissues. 



Subcrin is similar in many of its properties to cutin. It constitutes an 

 important part of cork cell walls and it is also found in the walls of a few 

 other specialized types of cells. Most of the surface of perennial plants, aside 

 from the leaves and very young stems is covered with suberized cell walls. 

 Such walls are relatively impermeable to water. The chemistry of both cutin 

 and suberin is discussed in Chap. XXIII. 



IlcmiccUuloscs are a poorly defined group of polysaccharides associated 

 with cellulose in plant cell walls. They are not chemically related to cellu- 

 lose, as the name implies, but possess very dififercnt chemical and physical 

 properties. The chemistry and metabolic significance of these substances are 

 discussed in Chap. XXII. 



Callose is the name given to a carbohydrate membrane substance found 

 in the perforated septa ("sieve plates") of the sieve tubes. Similar material, 

 presumably of the same chemical composition, has been found in pollen grains 

 and constitutes the inner layer of pollen tubes. It has also been reported as 

 occurring in the fungi. The exact chemical composition of callose is unknown 

 since it has never been obtained in sufficient amounts to permit quantitative 

 determinations. 



