354 SCIEXCE PROGRESS. 



There is great reason, howevtjr, to believe from Mangin's 

 researches that these also are formed largely if not entirely 

 of various pectic compounds. 



The various investigations that have been summarised 

 above, point unmistakably to the conclusion that the cell- 

 wall is originally far from homogeneous, and that while 

 cellulose enters very prominently into its composition there 

 are present in it a number of other substances which have 

 hitherto been somewhat loosely described under the names 

 of pectose, pectine and compounds of pectic acid. The 

 modifications of cell-wall which give rise to gums and 

 mucilages, all of which are probably very complex, may well 

 ■be derived from these and not from the cellulose constituent 

 at all. At the same time it must not be overlooked that 

 several modifications of so-called cellulose exist, differing 

 from each other possibly in the degree of their hydration. 



In considering- the composition of cell-wall, as far as it 

 has been determined at present, the researches of Mangin 

 (22) on the subject are most complete and of the first 

 importance. Till his memoir appeared the whole group of 

 pectic compounds was ill-defined, and nothing- very definite 

 was known as to the distribution in plant cell-walls of most 

 of them. 



These compounds can now be arranged in two series, 

 one of the latter comprising bodies of a neutral reaction, 

 while those of the other are feeble acids. In each there are 

 probably several members, which show among them every 

 stage of physical condition between absolute insolubility and 

 complete solubility in water, the intermediate bodies ex- 

 hibiting gelatinous stages, characterised by the power of 

 absorbing water in a greater or less degree. 



Of the neutral series the two extremes are presented by 

 Pectose and Pectine. The former is insoluble in water and 

 closely associated with cellulose in the substance of the 

 membranes ; the latter is soluble in water and forms a jelly 

 with more or less facility. 



In the other series the two most noteworthy members 

 are Pectic and Metapectic Acids. The former generally 

 exists in the membranes in combination with the metals of 



