156 



The Living Plant 



their various functions; and although innumerable forms occur 

 not directly referable to any of these types, they are never- 

 theless only modifications and combinations thereof. 



The cell-wall, however, is modifiable not only in shape, but also 

 in thickness. Ordinarily very thin, it can become thickened to 

 any degree required by function, even to the almost total ob- 

 literation of the cell cavity, as happens in some fibers (figure 50, d), 

 where the need for additional strength is perfectly plain : in cells 



*r 



FIG. 50. Various methods of adaptive thickening of cell-walls; further particulars in text. 

 (All copied from von Mohl's classical work on the Plant Cell, 1851.) 



devoted to the protection of something, notably in the shell of a 

 nut (figure 50, a) : and in cases where the formation of a thickened 

 wall is a means of storing additional food, as in the Ivory Palm and 

 Date (figure 36). A similar thickening is used also as a pro- 

 tection to the resting spores of Molds, Yeasts and disease germs, 

 which thereby are so completely protected against all hostile 

 outside influences that they can float uninjured for months in 

 the air, and germinate finally in the most unexpected and least 

 desired of places. In some cases the thickening is not at all uni- 

 form, but takes the form of rings and spirals, as in young ducts 

 which they help to keep open while the walls are still very flex- 

 ible (figure 50, 6) ; or it makes an elaborate fretwork of strength- 

 ening ridges surrounding thin areas easily pervious to water, 

 as in older ducts (figure 50, c) ; or it occurs upon one wall only, as 



