SECT, i MORPHOLOGY 69 



becomes club-shaped, and, by continued irregular deposits, it finally attains its 

 clustered form. 



So far only centripetal wall thickenings have been described. 

 Cells, the walls of which are centrifugally thickened, can naturally 

 only occur where the cell walls have free surfaces. The outer walls 

 of hairs generally show small inequalities and projections. The 

 surface walls of spores and pollen grains (Fig. 77) show a great 

 variety of such centrifugally developed protuberances, in the form of 

 spines, ridges, reticulations, and bands of characteristic structure. 



Cell-Wall Substance ( 51 ). The most important constituent of 

 cell Avails is CELLULOSE. It is present in the cell walls of all plants, 

 except in those of the majority of Fungi. 



Cellulose is a carbohydrate of which the chemical composition is expressed by 

 the general formula (C 6 H 10 5 )n. It is insoluble in dilute acids or alkalies. By 

 the action of concentrated sulphuric acid it is converted into dextrose. After 

 treatment with sulphuric or phosphoric acid, iodine will colour it blue ; it shows a 

 similar reaction when exposed to the simultaneous action of a concentrated solution 

 of certain salts, such as zinc chloride or aluminium chloride, and of iodine. 

 Accordingly, chlor-zinc-iodide, on account of the blue colour imparted by it, is 

 one of the most convenient tests for cellulose. GILSON ( M ) obtained cellulose in a 

 crystalline condition in the form of sphaerites or dendrites. 



The cell walls never consist entirely of pure cellulose, but contain 

 a considerable amount of other substances, which are not stained blue 

 by chloroiodide of zinc. In unlignified cell walls PECTIN SUBSTANCES 

 are particularly prominent. They are easily distinguished by the 

 readiness with which they dissolve in alkalies, after being previously 

 acted upon by a dilute acid. 



Susceptibility to certain stains for example, congo red is a characteristic of 

 cellulose ; while other stains, such as safranin and methylene blue, colour pectin 

 substances more deeply. According to MANGIX ( M ), the partition wall formed in 

 the higher plants during cell division consists almost wholly of pectin substances ; 

 the next developed laminae, the secondary cell- wall layer, of a mixture of cellulose 

 and pectin substances ; the last formed, or tertiary layer, chiefly of cellulose. If 

 the secondary layer of the cell wall remain unlignified, the amount of the pectin 

 substances contained in it increases with age and helps to strengthen the MIDDLE 

 i, \MKLLA, or primary cell- wall layer. 



Among the substances entering into the composition of cell walls, in addition to 

 cellulose and the pectin substances, mention must be made of CALLOSE. It is 

 characterised by its insolubility in cuprammonia and solubility in soda solution, 

 and in a cold 1 per cent solution of caustic potash. It is coloured a red brown by 

 chloroiodide of zinc, with aniline blue it takes an intense blue, and with corallin 

 (rosolic acid) a brilliant red. Its presence in the higher plants is limited to a few 

 special cases ; it envelops the sieve-pits and is always present in calcified cell-wall 

 layers, as, for example, in cystoliths (Fig. 76). Chitin, a proteid substance, 

 according to GILSON ( M ), takes the same place in the cell walls of the Fungi as 

 cellulose in the cell walls of the higher plants. 



Where cell walls become LIGNIFIED or SUBERISED, it is particularly 



