56 



THE CELL. 



in the stalks of many kinds of grasses and grains the outer cell- 

 walls become impregnated with solid deposits of silica, sometimes 

 to such an extent that the exact form of the cell is retained when 

 the tissue has been burned or the soft parts have been removed 

 by reagents. These siliceous deposits perhaps give to the stalks 

 the rigidity which enables them to maintain an erect position 

 against wind and showers. Siliceous deposits likewise occur in 

 the cell-walls of those microscopic one-celled plants known as 

 diatoms. So firm and enduring are the deposits, 

 that they are nearly indestructible by most 

 chemical and physical reagents. In certain parts 

 of the world great beds of earth occur which 

 consist almost entirely of the siliceous remains 

 of diatoms of bygone geologic ages. These are 

 so perfectly preserved that the various species 

 can be recognized as clearly as in the case of dia- 

 toms now living. Calcareous deposits occur 

 likewise in the cell-walls of many other plants, 

 and may be so extensive (as in the corallines) 

 that the whole plant appears hard and white, 

 as if sculptured out of marble. 



Among animals a similar process is com- 

 mon ; examples are the shells of snails, oysters, 

 and of other mollusks, the skeletons of corals, 

 and the calcareous and siliceous coverings of 

 a multitude of minute animals. These live 

 for the most part in the sea or in fresh water, 

 from which dissolved mineral matters are taken 

 and laid down in the solid form. As in the 

 case of diatoms, large beds of mineral matter 

 (e.g., chalk) have been formed by the accu- 

 mulated remains of these animals. Indeed 

 the recovery of solids from solutions is one 

 of the most important geological effects produced by living 

 things. 



In the last place, the cell-membrane may undergo great 

 changes in physical as well as chemical structure. These modifi- 

 cations are especially conspicuous and varied among plants. They 

 consist for the most part in the appearance of prominences or 

 depressions on either the outer or inner surface of the membrane. 



FIG. 26. (After 

 Sachs.) Part of a 

 long cylindrical 

 cell ("annular 

 vessel ") from the 

 woody tissue of 

 maize, showing 

 ring-like or " an- 

 nular" thicken- 

 ings ()) on the 

 inner surface of 

 the membrane. 



