78 HISTOLOGY AND PHYSIOLOOY. 



being ordinary wood or woody tissue, of indispensable use 

 in the industries of life. 



100. Growth of the cell-wall does not take place by the 

 addition or apposition of new layers concentrically around 

 the one first formed, as was for a long time believed. It 

 takes place, as is now quite generally held, by a process 

 called intussusception. This may be understood by a 

 reference to Fig. 181, which is a diagrammatic representa- 

 tion of the theoretical structure of the cell-wall. The small 

 squares represent the particles (inicellcB), or crystal-molecules 

 (aggregates of molecules of cellulose); 

 '^d'^q ^ h ^ r, °n°n ^n<i surrounding: each there are lay- 



c^a^° n° n n^o^ ersof water. The nutrient solution 



D n a D 



''□°D''r,°r,°ri'^n irom the protoplasm passes between 



D°D'^'^n°r, n'^n'^ the particles, and from it new parti- 

 °D°o'^n°n'^a°a °'°^' °'' ™i°^ll^) ^"S6 between those 

 D''o°"n° d'^d" already existing. It is evident that 



D Q '^ G D 



D|^DjjOj^p c^Djj growth can thus take place only 



, I, n. izi, when the cell-wall is in a turgid 



^^^ state, that is, distended by water. 



An increased amount of water pushing the already existing 



particles further asunder, affords space for the formation 



and existence of new ones. 



loi. The protoplasm is a very complex substance, 

 ■v^hose exact chemical composition has not been as yet 

 satisfactorily determined. It consists of albuminoids (con- 

 taining oxygen, hj'drogen, carbon, and nitrogen), with a 

 variable amount of water and a small amount of ash or 

 mineral constituents. It is transparent, often slightly 

 yellowish, and more or less granular under the microscojie. 

 Living protoplasm often exhibits movements both when 



Fig. 181. Diagrammatic representation of the minute structure of a cell-wall ; / 

 and ///, with less water; //, with more water surrounding the micellce. 



