THOMAS KERR 41 



or partly or completely neutralized by one or more bases as in calcium 

 pectate. The term "protopectin" is applied to the very long chained, 

 parent substance which occurs in primary walls. Protopectin is insoluble 

 in water, but upon restricted hydrolysis gives rise to pectic acid (free 

 of methyl groups) or pectinic acid (partly methylated). The intercellular 

 substance is commonly considered to be calcium pectate. 



It is possible to dissolve the cellulose of the primary wall and leave a 

 structural residue of the protopectin. Conversely the pectic substances 

 may be removed and there remains a structural framework of the 

 cellulose. One might expect that long chained molecules of protopectin 

 would be arranged in the primary membrane similar to the long chained 

 molecules of cellulose, but this apparently is not the case. The pectic 

 residue of the wall after the removal of cellulose is isotropic. At no time 

 do the pectic substances within the wall give an X-ray diffraction pattern, 

 and the removal of the pectic substances does not affect the diffraction 

 pattern which is already present. On the other hand, pectins removed 

 from the wall, formed into threads, stretched, and dried, will give an 

 X-ray diffraction pattern (8,10). Furthermore, Owens and his co-workers 

 have given excellent evidence to indicate that pectins possess rigid, 

 rod-hke molecules. The length of the protopectin chain is unknown but 

 pectinic acid derived from protopectin by hydrolysis has been reported 

 to have molecular lengths varying from 530 to 1650 A. (10). Considering 

 all these facts, the arrangement of the protopectin chains within the 

 wall is probably at random or the molecules might possibly be oriented 

 with respect to each other and yet incapable of crystallization. 



There are several other properties of pectic substances which are 

 important from the standpoint of their presence in primary walls, i) 

 Pectic substances are well known for the ease with which they form 

 gels. 2) In calcium pectate, the calcium ion is shared between two 

 carboxyl groups of adjacent chains of pectic acid, resulting in cross 

 linkages. 3) Pectins undergo degradation or depolymerization with ex- 

 treme ease when in aqueous solutions, and the degradation is markedly 

 influenced by various cations, particularly hydrogen ions. 



At least one property of the wall, rigidity, can be associated with the 

 properties of the pectic substances in the wall. Pickles of various kinds 

 from cucumbers, watermelon rinds, or green tomatoes are essentially 

 skeletons of the primary wall preserved in acid, usually after fermenta- 

 tion. During fermentation, cucumber pickles sometimes become soft, 



