DIV. r MORPHOLOGY 39 



without lessening the permeability of the wall to water and dissolved 

 substances. Corky and cutinised walls, on the other hand, are 

 relatively impermeable to water and gases, and greatly diminish 

 evaporation. The cell walls are frequently coloured dark by deriva- 

 tives of tannins, and thus, as in seed-eoats and in the old wood, are 

 protected against decay. In old cell walls inorganic substances often 

 accumulate in considerable amount. Silicic acid is frequent, calcium 

 carbonate less common, while organic salts such as calcium oxalate also 

 occur. 



LIGNIFICATION depends on the introduction into the carbohydrate layers of the 

 cell wall of various substances which are mainly benzole derivatives. The inner- 

 most layers of the wall of lignified cells consist, however, in many cases of cellulose. 

 Characteristic reactions for lignin are a yellow colour with acid aniline sulphate, 

 and a red colour with phloroglucin and hydrochloric acid. With chlor-zinc- 

 iodide lignified walls stain yellow, not blue. KLASON C 32 ) regards these reactions 

 as dependent on a condensation product of couiferyl- and oxyconiferyl- alcohol 

 which he calls lignin. 



SUBERISATION is as a rule limited to the middle thickening layers of a cell wall. 

 The corky lamellae consist of SUBERIN only and thus contain no carbohydrate. 

 CUTINISATION is closely related to suberisation but not identical. It consists in a 

 secondary deposit of CUTIN on a cellulose wall, or its introduction into the 

 substance of the wall. No sharp distinction can be drawn between cutin and 

 suberin. Both are coloured brownish yellow by chlor-zinc-iodide and take a nearly 

 identical yellow colour with potash ; they stain red with sudan-glycerine and are 

 both insoluble in concentrated sulphuric acid or ammonia-oxide of copper. Cutin, 

 however, resists the action of potash better. Both cutin and suberin behave 

 differently to reagents according to their special mode of origin. According to 

 VAN WISSELIXGH (^ suberin is a fatty substance which is composed of glycerine 

 esters and other compound esters of phellonic, suberic, and others of the higher fatty 

 acids ; the phellouic acid, which is a constant constituent of suberin, is wanting 

 in cutin. 



CALCIUM CARBONATE occurs in the walls of certain plants, e.g. of most Characeae, 

 in such amount that they become rigid and brittle. SILICIC ACID is present in the 

 peripheral cell walls of grasses, horse-tails, and many other plants (e.g. of the 

 unicellular diatoms), and makes them more rigid. CALCIUM OXALATE when present 

 is usually in the form of crystals. 



The pigments belonging to the flavone group which occur in the technically 

 valuable woods are also localised in the cell walls. 



Solid cell walls may undergo a transformation into GUM, as in the gummosis 

 of wood. In species of Pmnus and Citrus the thickening layers of the cell wall 

 become swollen one after another in this process, and ultimately the cell contents 

 are involved in the change ( 36 ). 



