76 



HISTOL 00 Y AND PHYSIOL OGY. 



forming a low dome (Figs. 176, 177) ; the overarching wall 

 does not meet in the middle ; and this 

 opening furnishes the inner ring, and 

 the bottom of the pit the outer (Fig. 

 177). Similar growth takes place 

 in corresponding sides of the wall, 

 and the thin partition soon breaks 

 away (Fig. 178), allowing free 

 communication between the cells. 

 Another method of thickening gives 

 rise to tissue, called collenchyma ; 

 here the corners of the cell alone 

 become thickened (Fig. 179). 

 ^™ g8. In cell-walls considerably 



thickened, a differentiation into numerous concentric lay- 

 ers is plainly manifest under the microscope (Fig. 180). 

 This is called stratification, and it is due to the fact 

 that alternate layers contain different amounts of water of 

 organization. The dark layers contain less, and the light 

 layers contain more, water. From a study of the stratifi- 

 cation in starch grains, Niigeli was led to the conclusion 

 that the molecules of the cell-wall are aggregated into 

 small particles in the form of crys- 

 tals (each of which he called a mi- 

 cella), each surrounded by a layer of 

 water. With larger amounts of water 

 these particles, or mieellce, are pushed 

 further apart, and the layer appears 

 light under the microscope. With a 

 decrease of the amount of water the 





Fig. 179. Collenchyma. Fig. 180. Hardened cells of the Pear, in which stratifi- 

 cation is shown. 



