INTERCELLULAR SUBSTANCES AND WALLS 



89 



Conclusions 



Their many variations not withstanding, certain basic themes 

 are clearly recorded in the constitution of walls and intercellular 

 substances. The very concept of an extracellular architecture based 

 upon organized macromolecules provides us with the most nearly 

 universal theme, and is in turn seconded by the concept of hetero- 

 geneity in the macromolecular building materials selected. 



In the recurrent cell-polysaccharide wall, cellulose, or chitin, 

 is accompanied by polymers of hexose and/or hexuronic acid. 

 In other cases, polysaccharide or polyhexosamine may be associ- 

 ated with protein. Such combinations exist in bacteria yeasts, 

 arthropods, and vertebrates. Commonly, these hydrophilic materials 

 are covered by or encrusted with resistant substances, such as 

 waxes, lipids, or minerals. Mechanical adaptations involve the 

 incorporation of substances such as lignin or mineral. In all of 

 these instances, an ordered high polymer is laid down, infiltrated 

 by lower molecular weight ground substances, and finally modified 

 (and perhaps stabilized) by cross-linking agents, hydrophobic 

 substances, or minerals. 



Thus, even in this modest sample of the biological domain, 

 we can begin to develop some picture of the way in which chemical 

 capabilities, macromolecular properties, and surface conditions 

 together are reflected in the substances that organisms secrete or 

 elaborate external to their protoplast membranes. 



