20 OEGANOGEAPHY. 



observed more or less in all cells where active chemical changes 

 are going on ; thus it may be especially seen in those of the 



Fig. 34. 



Fig. 35. 



Fig. 34. Transverse section of 

 the testa of Datura Strarno- 

 nium. H. B. Brady. 



Fig. 35. Transverse section of 

 a thick-walled cell of the 

 pith of Hoya carnosa. From 

 Mohl. 



/pith of Ho^a carnosa (Jig. 35). These thickening layers are 

 commonly called secondary layers or deposits. It is these de- 

 posits which give hardness and firmness to the wood of plants 

 and to the stones of fruits, and hence the name of ^clerogen 

 (from a Greek word signifying hardness) has been given to tFem. 

 / The term Lignin is also frequently applied to them from their 

 / common occurrence in wood. Lignin is said to be a mixture of 

 several proximate principles. Like cellulose, however, it is com- 

 posed of three elements, carbon, oxygen, and hydrogen, but the 

 proportion of hydrogen is greater in it than in cellulose. Lignin 

 is insoluble in water, but always soluble in alkaline liquids. It 

 is rarely, or ever, deposited in a pure state. 



Pitted or Dotted Cells. — In almost all cases when the cell- 

 membrane has thus become thickened by secondary layers, it 

 presents (instead of the smooth and homogeneous appearance, as 

 is the case, as we have seen, when it is in a young condition) a 

 greater or less number of dots, pits, or slits of various kinds (Jigs. 

 36 and 37, e). These dots, &c. were formerly considered as actual 

 openings in the walls of the cells, and hence such cells were 

 called^orows cells ; but, when carefully examined, it may be readily 



