CELL-CONTENTS AND FORMS OF CELLS. 255 



the glandular hairs of kamala, or it may be changed into mucilage, 

 as in chondrus, or transformed into pectin compounds, as in fleshy 

 roots and fruits. 



To this middle plate is added on either side by the new.ly 

 formed protoplasts a layer of substance closely resembling cellu- 

 lose, this constituting the PRIMARY MEMBRANE or primary lamella. 



As the cells become older the wall increases in thickness 

 through the addition of other layers, much in the same manner as 

 the starch grain increases in size. These subsequent layers are 

 known as SECONDARY LAMELLAE. In a few cells the secondary 

 lamellae may consist of pure cellulose. As a rule, however, the 

 wall is rather complex and consists of alternate layers of cellulose 

 with other substances. Some of these, as mucilage, may be a 

 simple modification of cellulose, others may consist of cellulose 

 in combination with other substances, as in the ligno-cellulose walls 

 of stone cells, or the walls may consist of cellulose and suberin as 

 in cork cells, or of cutin-cellulose as in epidermal cells. Again, 

 there may be, through the action of special enzymes, a decomposi- 

 tion of the cellulose, resulting in the formation of oils, resin, and 

 wax. Furthermore, it would not seem improbable that some of 

 the secondary substances in the wall are direct products of the 

 protoplasm and secreted in the cell-wall, as silica and calcium 

 oxalate in epidermal cells. The substance called pectin originates 

 as a modification of the intercellular substance, and is peculiar 

 to some fruits. 



As showing, to some extent, the complexity of the lamellae 

 in the cell-wall, the following modifications of the wall in secretion 

 cells may be given: I. The entire outer cell-wall may consist of 

 a thin layer of suberized lamellae, beneath which is a secondary 

 mucilaginous layer that develps the secretion, e.g., Hedychium 

 Gardnerianum. 2. The outer lamellae of the cell-wall may con- 

 sist of suberin; beneath this. is a cellulose lamellae, which only 

 after treatment with a solution of potassium hydroxide is colored 

 blue with chlor-zinc iodide; this is followed by a mucilaginous 

 layer, e.g., Laurus nobilis, Curcuma Zedoaria, Cinnamomum Cas- 

 sia, Zingiber officinale, Acorus Calamus. 3. The outer layer may 

 consist of cork, beneath which is a cellulose layer that is colored 

 blue upon treatment with chlor-zinc iodide without the previous 



