60 THE ELEMENTS OF VEGETABLE HISTOLOGY 



substances and in some instances appear to be mix- 

 tures, definite reactions can seldom be given. Pectins 

 or mixtures of pectic substances are widely distributed 

 in the cell walls of fruits. This substance is responsible 

 for the hardening or " jelling " of fruit jellies. 



1. The fact that gums swell when brought into contact with 



water is of value in demonstrating the presence of these 

 substances in the cell wall. The material should be 

 mounted in absolute alcohol and the water gradually 

 added. 



2. Pectic substances are best identified by their reactions with 



various aniline dyes. Fuchsin, methylene blue and 

 Bismarck brown stain pectic materials as well as lignin 

 and suberin; but the latter substances retain the color 

 after treatment with alcohol or acids, whereas the pectic 

 substances are decolorized by this treatment. 



CHEMICAL PROPERTIES OF CELL CONTENTS 



The protoplasm, or material within the wall of a 

 vegetable cell, contains structures concerned in the 

 physiological activity of the cell together with inclu- 

 sions, or substances produced by this protoplasm. 

 The term cell contents might be used for all materials 

 within the cell wall, thus including such physiological 

 parts as the nucleus and vacuoles, together with starch, 

 inulin and other inclusions. However, in vegetable 

 histology the term cell contents is restricted to materials 

 resulting from the activity of the protoplasm and would 

 therefore include starch grains, calcium oxalate crys- 

 tals and other substances. These inclusions, or non- 

 protoplasmic cell contents, may be inorganic or organic 

 substances. They are most conveniently subdivided 

 into substances of definite form and those of indefinite 

 form. The histology of the various cell contents will 

 be considered in a subsequent section but their chemical 



