THE CELL. 77 



particles approach each other, making a dense layer, and 

 appearing dark under the microscope. Sometimes a system 

 of layers at right angles to the layers of stratification is 

 evident, due likewise to varying amounts of water con- 

 tained. This is called striation. Owing to chemical 

 changes a cell-wall may actually become separated into two 

 or more layers or shells, as in pollen grains, spores, etc. ; 

 but this has no necessary connection with stratificatiou 

 or striation. 



gg. The cell-wall is composed of cellulose (which con- 

 sists of carbon, hydrogen, and oxygen, and whose formula 

 is Ci2 II20 Oio), water of organization, and ash constituents. 

 The ash or mineral constituents constitute often less than 

 one per cent, of the whole weight of the plant. Three to 

 five per cent, may be considered the average, though it may 

 run as high as twenty or thirty per cent. Potassium, 

 calcium, magnesium, iron, ph,osphorous, sulphur, sodium 

 (traces of manganese), silicon, and chlorine exist in the 

 ash. In addition to these, bromine and iodine are found 

 in the tissue of marine plants. The chemical changes, or 

 metamorphoses, which the cell-wall often undergoes are (1) 

 conversion into mucilage ; (2) conversion into cork ; and 

 (8) conversion into wood, or lignification (Lat. lignum, 

 wood). The seeds of Flax and Quince furnish an example 

 of the first. Corky layers are developed abundantly in 

 some plants, as in the Cork-Oak. Cork is impervious to 

 water. When a plant is wounded slightly on the surface, 

 layers of cork are developed immediately under the wound, 

 and thus protection is afforded to the parts bfeneath. Lig- 

 nification of the cell-walls takes place in all but herb- 

 aceous plants, and consists in the impregnation of the 

 cellulose, with a substance called lignin ; the tissue resulting 



