64 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



stances leads one to realize that their constitution and 

 physiological functions are as yet very imperfectly under- 

 stood. 



The middle lamella of cell walls consists of pectose, or 

 possibly of calcium pectate. Pectose can be distinguished 

 from cellulose by its insolubility in ammoniacal cupric 

 hydroxide, and by the fact that when treated with hot 2 per 

 cent, hydrochloric acid followed by sodium hydroxide it goes 

 into solution. This pectose is by some considered to be a 

 calcium salt of pectin, but the fact that it requires some- 

 what prolonged treatment with acid to bring it into the 

 condition in which it dissolves in an alkaline solution with 

 formation of a salt seems to weigh against such a view. 

 It is accordingly more probable that the change from 

 pectose to pectin involves hydrolysis. During the ripen- 

 ing of fruits and the retting of flax, the breaking down of 

 the middle lamella of pectose is said to be brought about 

 by an enzyme, pectosinase. The pectin produced in this 

 reaction may be precipitated by alcohol or by calcium salts, 

 to form in the latter instance a calcium pectinate which 

 is soluble in 0-2 per cent, hydrochloric acid. But when 

 pectin is acted on by pectase, a substance, one of the many 

 pectic acids, is produced, the coagulation of which is in- 

 duced by the presence of calcium salts; this gelatinous 

 material is not dissolved by the previously mentioned con- 

 centration of acid. 



The end product, obtained by heating these pectic sub- 

 stances with hot dilute alkali, is arabinic acid. When 

 hydrolyzed with dilute acid they all yield d-galactose and 

 Z-arabinose, the proportions of the two varying with the 

 kind of pectin. In the cell this change is supposed to take 

 place through the activity of yet another enzyme, pectinase. 

 The careful analyses of pectin from different sources 

 which havejbeenjperformed in recent years by Tromp de 



