i6 



LECTURE II. 



to take place by the intercalation of new solid particles be- 

 tween those already in existence, a mode of growth which, he 

 has termed growth by intussusception. Strasburger is, however, 

 of opinion that such an intercalation of new solid particles 

 does not take place. He regards the subsequent increase in 

 surface of the cell-wall to be either merely a phenomenon of 

 imbibition, or to be due to stretching. 



The growth in thickness of cell-walls, and, we may add, 

 the increase in bulk of starch-grains, is brought about, ac- 

 cording to Dippel, Schmitz, Strasburger, and others, by the 

 repeated formation of laminae, of cellulose in the one case 

 and of starch in the other, in the manner described above. 

 These laminae are deposited, in the case of cell-walls, on the 

 inside of those previously formed ; in the case of starch- 

 grains, on the exterior of the grain. This mode of growth is 

 termed growth by apposition. 



A thickened cell-wall always exhibits evident hetero- 

 geneity of structure. The wall of a bast-fibre, for instance, 

 seen from the surface, presents a number of lines crossing it 

 obliquely or even at right angles to its long axis ; these con- 



FIG. 7. 



FIG. 8. 



FIG. 7 (after Naegeli). Cell-wall seen from the surface, shewing striation : 

 A oblique, B transverse ; a, lumen of cell. 



FIG. 8 (after Sachs). Transverse section of cell-wall, shewing stratification: 

 j/, split of the wall into two shells ; K, canals traversing its thickness ; 

 /, lumen of cell. 



stitute what is known as the striation of the cell-wall (Fig. 7). 

 Seen in transverse section the wall presents a number of con- 

 centric layers which constitute its stratification (Fig. 8). Naegeli 



