Microfibrils are composed of cellulose, possibly with small amounts of entrapped hemicelluloses. In aerial organs (stems, hypocotyls, leaves), the orientation of microfibrils in the outer wall of the epidermis governs the direction of organ growth (length versus girth), this specific wall being much thicker than the others in a growing organ (Kutschera 2008).

Cellulose can be partially purified as the 'a-cellulose' fraction - wall material left insoluble after exhaustive extraction of the matrix polysaccharides e.g. with 6 M NaOH at 37 °C. Prepared in this way, cellulose is contaminated by glycoproteins (especially extensins) and some pectic material (possibly covalently linked to the extensins).

Cellulose is relatively resistant to hydrolysis in 2 M TFA at 120 °C for 1 h, although a small proportion can be released as glucose, especially if the crystallinity of the cellulose has been compromised by severe alkali pretreat-ments. Good conversion of cellulose to glucose can be achieved by a two-stage acid hydrolysis (Saeman method: stirring in 72% w/w H2SO4 at room temperature to dissolve the polysaccharide, then heating in 2 M H-SO4 at 120 °C for 1h to hydrolyse it; subsequently the H2SO4 is neutralized with a small excess of BaCO- - . More convenient, however, is digestion of the a-cellulose fraction with Driselase, which usually gives an excellent yield of glucose. A short-cut, omitting the preparation of a - cellulose, is sequential treatment of cell walls with 2 M TFA (120 °C, 1 h) and then Driselase, giving a useful estimate of non-cellulosic and cellulosic Glc residues respectively (O ' Looney & Fry 2005; Fig. 1.2); note, however, that contaminating starch would be recorded as non-cellulosic Glc in this procedure.

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