THE PLANT-CELL. 15 



water or imbibition by the cell-wall has come to be termed its 

 "swelling-up." 



The cell-wall possesses, further, well-defined optical pro- 

 perties. When examined with polarised light it is found to 

 be doubly refractive, and, as von Mohl pointed out, its refrac- 

 tion is negative (Figs. 4 and 5). 



FIG. 5. FIG. 6. 



FIG. 5 (after Dippel). Transverse section of a tracheide of Pinus sylvestris viewed 

 with crossed Nicols. 



FIG. 6 (after Richter). Cystolith of Ruellia picta viewed with crossed Nicols. 



Accepting for the present this brief statement of the 

 chemical and physical properties of the cell-wall in its simplest 

 form without explanation, we may pass on to consider firstly 

 its formation, and secondly its growth and the chemical 

 and physical changes which may accompany it. 



With regard to the first formation of the cell-wall, it is 

 usually considered that the cellulose is secreted by the proto- 

 plasm in the form of a membrane either over its whole surface, 

 as in the case of isolated cells, or in the plane of division, 

 as in the case of cells forming part of a tissue. Schmitz 

 and Strasburger, however, are of opinion that the cell-wall is 

 formed by the actual conversion of a layer of protoplasm into 

 cellulose. Further details must be reserved until we have 

 become acquainted with the structure of protoplasm. 



The growth in surface of cell-walls is considered by Naegeli 



