THE CELL. 31 



meability to water. A fatty substance called " suberin " (HOHNEL) 

 (cutin) is contained in the cell-walls thus changed. Concentrated 

 sulphuric acid does not dissolve corky cell-walls. Continued boil- 

 ing with " Schulze's mixture," that is, chloride of potassium and 

 sulphuric acid (caution ! ), causes the suberized membranes to form 

 into oily drops of eerie acid. These corky membranes are widely dis- 

 tributed. The teleological explanation of this is that cells exposed 

 to the air require such membranes to guard against excessive evap- 

 oration. Examples: epidermal cells, pollen-grains, spores, etc. In 

 connection with the epidermal tissue we will refer to the waxy 

 deposits on the outer surface of the cell-wall. (Great extensibility 

 is not characteristic of corky cellulose membranes, as is frequently 

 maintained.) 



(b) Lignification cannot be satisfactorily explained at present. 

 Microchemistry and analytical chemistry have explained many 

 things, but have failed to explain definitely what lignification 

 is or what function it serves. A well-known reaction of woody 

 membranes is a red coloration with phloroglucin and hydrochloric 

 acid. Anilin sulphate colors them yellow ; phenol with hydro- 

 chloric acid stains them green to blue. Woody membranes resist 

 the action of sulphuric acid more than cellulose and less than corky 

 membranes ; with iodine and sulphuric acid they turn yellow or 

 yellowish brown. After treatment with potassium hydrate the 

 above-mentioned cellulose reaction (blue coloration similar to that of 

 fungus cellulose) appears. The chemical nature of woody cell-walls 

 has been studied by various authors, among whom are HOHNEL, 

 SINGER, and NICKEL. The red reaction with phloroglucin and 

 hydrochloric acid is probably due to two substances, coniferin (lig- 

 nin) and vanillin. NAGELI believed the chief cause of lignification, 

 to be a deposition of mineral salts (lime salts). The physical prop- 

 erties of woody cell-walls also require further study. 



(c) There is a modification of the cell-membrane remarkable for 

 its power of absorbing large quantities of water with considerable in- 

 crease in volume ; in the dry state it is hard and brittle, when filled 

 with water it is mucilaginous, hence the designation mucilaginous 

 cells. We are here concerned with various gums and plant mucilages, 

 some of which give a cellulose reaction with iodine and sulphuric 

 acid, while others do not. Such membranes serve to retain the mois- 

 ture for the plant. Medicine utilizes the mucilaginous products of 

 various plants. Of these may be mentioned the gelatinous stalks of 



