THE CELL. 27 



■wliich are to be considered in regard to the structure and growth 

 of starch grains also apply to the cell-wall. Excessive drying of the 

 cell-wall causes the stratification to become indistinct or to disappear 

 entirely, but it will reappear on the absorption — imbibition — of 

 water. This proves that the stratification of cell-walls depends 

 upon differences in the amount of water. The same is true in 

 regard to striation. Stratification also disappears when an excessive 

 amount of water is taken up by imbibition. 



Nageli applied his theory of growth by intussusception to 

 the stratification of starch-grains and cell-walls. The opposition 

 to this theory still continues : scholars are divided into two 

 distinct groups. Although Nageli's work in regard to starch- 

 grains is one of the greatest and most important productions in 

 botanical science, yet the fact remains that the process of apposition, 

 at least in regard to the growth of cell-walls, is of a wider applica- 

 tion than N^iigeli's theory would seem to permit. A refutation of 

 the theory of intussusception is nevertheless out of the question. 



According to the theory of intussusception the starch-grain 

 within the cell increases in size at the expense of the soluble starch 

 substance entering through the cell-wall fi-om without, assisted by 

 the living plasm of the cell. The cell-wall receives its building 

 material direct from the primordial utricle. The apposition theory 

 teaches that the strata are formed by superposition, always on 

 the outer surface of the starch-grain, and on the inner surface 

 of the cell-wall. There is no doubt that the lamellae of the cell-wall 

 are frequently formed by apposition, but the growth of such lamellce 

 is evidence that their increase in thicltness takes place internally, 

 and not on the surface. In proof of this there are certain facts 

 concerning development to be mentioned below. Such facts, how- 

 ever, are also evidence of surface growth by intussusception. I will 

 limit myself to the following statements. 



The development of the algal group Glceocapsa gives additional 

 evidence in favor of the growth in thickness and surface of cell-wall 

 according to the theory of intussusception (ISTageli,' Correns"). 

 The outer cell- wall grows in thickness and in surface until it has 

 increased 219 times its original volume without being in direct con- 

 tact with the cell-plasm. Although this example holds true for 



' Stark ekorner. 



' Flora, 1889. Also Wille's contributions in defence of the theory of intussus- 

 ception (Christiania, 1886). 



