70 BOTANY I-AKT i 



the secondary layer that receives the wood or cork substance, while 

 the tertiary or internal layer retains its cellulose character. 



Lignificatiou depends on the introduction into the membrane of certain 

 substances, among which, according to CZAPEK (**), an aromatic aldehyde which 

 he names hadromal is never wanting. According to F. C. vox FAIJEU ("*) hadromal 

 is not present in all lignified membranes, and on the other hand occurs in many 

 that are not lignified. Associated with hadromal, according to CZAPEK, is coniferin, 

 whieh can be obtained from the youngest xylem. C/AP-EK denies 1 1n-, existence of 

 vanillin in lignified membranes. The proportion in whicli hadromal is found in 

 wood does not exceed 1-2 per cent of its dry weight. To its presence the so-callcil 

 liguin reactions are due, a violet colouration with phloroglucin and hydrochloric 

 acid, and a yellow colouration with acid anilin sulphate. The potassium per- 

 manganate reaction (a red colouration on treatment with a 1 per cent solution of 

 that substance followed by ammonia) is, according to F. C. FABER, a general feature 

 of lignified membranes. With chlor-zinc-iodide lignified membranes stain yellow, 

 not blue. 



Corky cell walls contain suberin and take a yellowish-brown colour with chloro- 

 iodide of zinc ; with caustic potash, a yellow. VAN AVissF.r.iNiac ( s ~) has disputed 

 the presence of cellulose in suberised cell walls, and regards the cork substance or 

 .sruKUix as a fatty body, which is composed of glycerine esters and other com- 

 pound esters, as well as of one or more other substances which are infusible, insoluble 

 in chloroform, and decomposed by a solution of caustic potash. 



CUTINISATION, which is similar to but not identical with suberisa- 

 tion, is usually due to the subsequent deposition of cutin in cellulose 

 cell walls. 



VAN "\VI.SSKI.INGH has shown that phellonic acid, which is always present in 

 suberiu, is constantly absent in cutin. The behaviour of cutin, as of suberin, varies 

 according to the source from whicli it is derived. Cutin withstands better the 

 action of- caustic potash. In other respects, the reactions given by cutinised cell 

 w r alls with chloroiodide of zinc or solutions of caustic potash are almost identical 

 with those of suberised cell walls. 



Young cell walls are less elastic, but relatively more extensible 

 than older ones. The power of resisting a stress is increased by 

 lignification. The presence of cutinised and corky membranes at the 

 surface of the plant diminishes the loss of water from it ( M 



The layers of the cell walls of some cells, particularly the super- 

 ficial cells of certain fruits, as of Sage, and of numerous seeds, such as 

 Flax and Quince seeds, become mucilaginous, ;tnd swell in water to 

 MUCILAGE, which, according to G. KLEBS ( 59 ), serves the purpose of 

 attaching the seeds to the soil. Firm cell walls can also be trans 

 formed into GUM, as is so often apparent in Cherry and Acacia trees, 

 portions of the wood of which often succumb to GU.MMUSIS. 



The several varieties of gvms and mucilage react differently, according as they 

 are derived from cellulose, callose, pectin substances, or from allied sulistam -e*. 

 According to MANC.IN they may be microchemically distinguished by their ivai-tit>n 

 with ruthenium red, which stains only such as are derived from pectin -nlt.un .- 



