THICKENINGS OF THE WALL. 31 



of the cell-wall was for a long time not fully understood. 

 There have been three prominent theories advanced to ex- 

 plain the phenomena observed. They may be briefly stated 

 as follows : 



I. Von Mohl held that " the growth of the cell-membrane 

 in thickness arises from a periodical apposition of new mem- 

 branes upon the already completely developed wall."* Ac- 

 cording to this theory, the marks of stratification usually seen 

 were supposed to be the lines separating the added mem- 

 branes. This deposition was supposed to proceed from with- 

 out inwards ; that is, the newer layers were supposed to be 

 placed inside of the previously existing ones ; on this ac- 

 count this has been called the theory of centripetal thicken- 

 ing. Until quite recently this has been the prevailing theo- 

 ry in English and American books. 



II. Some observers, among whom were Hartig and Hart- 

 ing, laying great stress upon the external markings, as seen 

 in pollen grains, spores, etc., opposed the foregoing theory, 

 and propounded one which has been termed the theory of 

 centrifugal thickening. According to this theory, "the cell- 

 membrane increases in thickness in the direction from 

 within outwards by the deposition of layers upon the out- 

 side of the original membrane." It is thus the exact oppo- 

 site of the previous one ; while in the former the outer 

 membrane is supposed to be the oldest, in the latter it is the 

 inner one. 



III. The theory which now generally prevails is that the 

 thickening of the wall is a growth, due to the formation or 

 deposition of new molecules between the molecules of the 

 original membrane. It is called the theory of intussuscep- 

 tion, and was originated by Nageli in 1858. f 



* The student will find a condensed statement of this theory in the 

 " Principles of the Anatomy and Physiology of the Vegetable Cell," by 

 Hugo Von Mohl, translated by Henfrey, 1851. 



f Nageli, " Die Starkeko'rner," in " Pflanzenphysiologischen Unter- 

 euchungen," 1858. Duchartre claims for Trecul the first suggestion of 

 this theory in 1854. The term intussusception as applied to the growth 

 of the cell-wall was used long before this ; Schleiden, in his " Contri- 



