CONSTITUTION AND ARCHITECTURE IN THE CELL WALL 31 



calcium, magnesium, phosphorus, and silicon. In some plant 

 groups relatively high inorganic residues containing considerable 

 proportions of Si0 2 are to be found. Sorghum, wheat, and maize 

 are representative of the Graminae which contain 10-12 per cent 

 ash, principally silica. Sorghum and wheat are extreme cases, 

 with inorganic residues running 90 per cent Si0 2 . Among 

 dicotyledonous plants, sunflower and Lantana contain about 11 

 per cent ash of which onequarter is SiC»2. Equisetum is notable 

 among the non-flowering vascular plants with its silica content 

 of 8-10 per cent. 



The study of mineral (ash) constituents of cells involves the 

 technique of microincineration. In general, longitudinal or trans- 

 verse sections, or whole tissue if sufficiently thin are pressed 

 between microscope slides and ashed at 450-900°C. The upper 

 slide is then removed, Canada balsam added, and the cleared 

 material placed under a standard cover glass. The resulting mounted 

 residue from incinerated tissue is known as a spodogram. 



By this procedure, mineral substances are frequently retained 

 in their natural position in the cell. In spite of the dimensional 

 changes which may occur, relative locations (for example, nuclear 

 vs. cell wall residues) may be distinguished. Both microchemical 

 and microphysical methods, including petrographic and crystallo- 

 graphic analysis may be applied to these inorganic components 

 of tissues, cells, or parts of cells. 



The siliceous residue may vary considerably in particle size 

 and shape. In the sorghum leaf, irregularly shaped particles meas- 

 uring approximately 0.007 x 0.01 52 mm; dumb-bell shaped par- 

 ticles measuring 0.0135 < 0.0207 mm; and rectangular particles 

 0.017-0.073 mm long may be found. The leaf sheath in wheat 

 contains oval particles 0.017x0.0153 mm. 



In bamboo, the nodes contain particles 0.41-0.57 mm in length. 

 The deposition of silica in Lantana is restricted to trichomes and 

 epidermal cell walls in the lamina. The silicified trichome measures 

 at maximum, about 0.5 mm in length. The irregular particles in the 

 sunflower have a maximum diameter of about 0.06 mm. The ash 

 of sorghum, wheat, maize, sunflower and bamboo contain an 

 isotropic silica with an index of refraction of 1.45. 



