THE CELL-WALL. 



23 



26. Growth in thickness of the wall produces changes 

 in the cell of even greater importance than growth in sur- 

 face. While surface-growth has but little to do with the 

 determination of the functions of the cell, the thickening of 

 its wall generally results in a 

 change in function, or an entire 

 suspension of all physiological 

 activities. Cells with extremely 

 thin walls are most active ; only 

 such can take part m growth. 

 (See Chap. XL) Nutrition and 

 assimilation are confined to cells 

 whose Avails have but slight thick- 

 ness. Cells Avith moderately thick 

 Avails may be used as storehouses 

 for food ; starch, for example, is ><"'- After Duchanre. 

 frequently found in such cells. But as the walls attain great 

 thickness the protoplasm loses all activity save that neces- 

 sary to the secretion of cellulose. 



27. The thickening generally produces certain markings 

 or sculpturings in the shape of projecting points, ridges, 

 bands, etc., which on the one hand are on 

 the outside of the wall, \vhile on the 

 other they are on the inside. In some 

 pollen grains and spores Ave have the best 

 examples of external markings. Here, in 

 some cases, certain isolated points in the 

 cell-wall become strongly thickened, giv- 

 ing rise to spines or prickles (Fig. 11). 

 In other cases the thickening is in cer- 

 iV fur ished with ridge- tain bands, which may rise into high 



like thickenings united __' . , . 



into a network. Each of Avails, as in Fig. 12. External markings 



these beirs thickenings, , ,. , . , 



wh<ch project stm more, occur only upon cells Avhich are tree, or 



in the forai of spines ar- i , , ,> i 



ranged like a comb. After in slight contact Avith one another or 



with other cells. 



28. Internal markings are of essentially the same kind 

 as the external, although of greater variety. When the 

 secretion of new cellulose is greatest at isolated points, knobs 

 and projections of various kinds are the result. It more 



