32 BOTANY. 



37. Every part of the living cell-wall appears, from the 

 results of Nageli's researches, to be composed of definite 

 molecules, which are not in contact, but separated from one 

 another by layers of water, termed the Water of Organiza- 

 tion. The thickness of these intermolecular layers, and con- 

 sequently the amount of water in the whole mass of any cell- 

 wall, varies in different cells, and even in the same cell. In 

 the denser walls, or parts of walls, the water is less ; in those 

 which are less dense it is greater. (Fig. 22.) 



Now it is evident that young cell-walls must have rela- 

 tively large amounts of water in their substance, and here is 

 where we find a growth taking place. Sachs supposes* that 

 an aqueous solution derived from the protoplasm penetrates 

 by diffusion betAveen the molecules of the cell-wall. This is 

 not a solution of protoplasm, but probably some carbohy- 

 drate constituent of the protoplasm which is easily trans- 

 formed into cellulose. From this nutrient solution there 

 may be formed in the spaces filled with water new molecules 

 of cellulose, which push aside and separate the previously 

 formed ones ; or the previously formed molecules may be 

 simply enlarged by the apposition of new matter. 



According to the theory just described, the formation of any projec- 

 tion upon the inner surface of the cell-wall is not by the superficial 

 deposition of molecules upon any definite area of the surface of the 

 wall, but by the abundant and continued deposition of new molecules 

 in the wall ; it consequently becomes thicker at the place of deposi- 

 tion ; in this thickened portion still more molecules are deposited, and 

 the thickness is further increased, and so on. In the same way projec- 

 tions are formed upon the outside of the wall by a slow internal growth. 



38. Stratification of the Wall. During the increase of 

 the cell-wall in thickness, an appearance of stratification 

 arises in it (Fig. 23). A cell-wall in which this is strongly 

 developed appears to be made up of concentric layers, and 

 this no doubt gave rise to the two theories before men- 



butions to Phytogenesis," 1838, makes use of the word, but it may be 

 doubted whether he or Trecul gave it exactly the meaning we now do. 

 * "Lehrbuch," fourth edition, and the English translation of the 

 third edition (" Text-Book of Botany "), Books I. and III. 



