THE OELL-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 walls have but slight thick- 

 ness. Cells with moderately thick 



walls mav hp iisprl ns sfriwlimisps Mg. 11.— Pollpn grain of Xamfem 

 wailb may oe USea as Storenouses tHmestris, covered ■ witn prickles. 



for food; starch, for example, is x m- After Dachanre. 

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

 thickness the protojDlasm 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, while on the 

 other they are on the inside. In some 

 pollen grains and spores we have the best 

 examples of external markinss. Here, in 

 some cases, certain isolated points in the 

 cell-wall become strongly thickened, giv- 

 JJiJ'-o^^ruJin!^- ing rise to spines or prickles (Pig. 11). 

 5m«. The almost spherical jn other cascs the thickening is in cer- 



substance of the ci'll-wall . ° . t . , 



is furi ished with ridge- tain bands, whicli may rise into high 



like thickenings united „ . ' . ^_ _■' . , , . ° 



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



these bears thickeuings, , ,, i • i ^ 



which project siiii more, occur Only upon cclls which are tree, or 

 rauKediiScomb!-ifter in slight contact with 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 

 iind projections of various kinds are the result. It more 



