MORPHOLOGY OF HIGHER PLANTS. 183 



Protective cellulose walls are composed of mixtures of lig- 

 nocellulose and oils and waxes, and frequently contain in addi- 

 tion tannin, vanillin and other compounds. In the cuticle or epi- 

 dermis of leaves and green stems, the cellulose is associated with 

 a fatty compound known as cutin (or cutose), while in the cork 

 of stems and roots it is combined with suberin (or suberose). 

 This class of celluloses is distinguished from cotton cellulose and 

 lignocellulose by being insoluble in sulphuric acid. 



Reserve cellulose walls are those found in various seeds, as 

 in coffee, date, nux vomica, etc. They behave toward reagents 

 much like the true celluloses. 



Mucilage cellulose walls consist of cellulose and mucilage 

 and are found in all parts of the plant, and in the case of seeds 

 are associated with the protective celluloses. They dissolve or 

 swell in water, are colored blue or yellowish with iodine, and are 

 stained with alcoholic or glycerin solutions of methylene blue. 



Mineral cellulose walls are composed of cellulose and vari- 

 ous inorganic substances, as silica, calcium oxalate or calcium 

 carbonate. These are more commonly found in the cell wall of 

 the lower plants, as Algae, Fungi and Equisetacese. Calcium car- 

 bonate also occurs in the cystoliths of the various genera of the 

 Woraceae and Acanthaceae (Fig. 221). 



TTom what has just been said of the chemical composition and 

 structure of the cell wall, it is seen that it consists of lamellae or 

 layers of different substances, and in no case does it consist of 

 but a single substance ; but for convenience we speak of a wall as 

 consisting of cellulose, lignin, or suberin, meaning thereby that 

 the wall gives characteristic reactions for these substances. 



In some cells, as in lignified cells, the lamellae are quite appar- 

 ent, whereas in other cases the use of reagents, as chromic acid, 

 or chlor-zinc-iodide, is necessary to bring out this structure. The 

 layering which is observed in transverse sections of the cell wall 

 is, spoken of as stratification of the wall, whereas the layering 

 observed in longitudinal or tangential sections is referred to as 

 striation of the wall (Fig. 299, B). 



Thickening or Marking of Walls. — In the formation of 

 the cell wall each cell appears to work in unison with its fteigh- 

 bors for the building up of the plant. The thickening of the walls 



