78 HISTOLOGY AND PHYSIOLOGY. 



being ordinary wood or woody tissue, of indispensable use 

 iu 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 (micellae), or crystal-molecules 



(aggregates of molecules of cellulose) ; 

 and surrounding each there are lay- 

 ers of water. The nutrient solution 

 from the protoplasm passes between 

 the particles, and from it new parti- 

 cles, or micellae, arise between those 

 already existing. It is evident that 

 growth can thus take place only 

 when the cell-wall is in a turgid 

 Btate, 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. 



101. The protoplasm is a very complex substance, 

 whose exact chemical composition has not been as yet 

 satisfactorily determined. It consists of albumiuoids (con- 

 taining oxygen, hydrogen, carbon, and nitrogen), with a 

 variable amount of water and a small amount of ash or 

 mineral con'stituents. It is transparent, often slightly 

 yellowish, and more or less granular under the microscope. 

 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 micelleE. 



