22 Plant Physiology 



tallization of the pigment contained. The special signifi- 

 cance of these three types of bodies will require treatment 

 later. 



15. The cell-wall. The plant protoplast is commonly, 

 and in the vegetative organs of higher plants invariably, 

 invested by a firm cell-wall. When constituting a part 

 of a tissue-system, the cell-walls are throughout most of 

 their length in close contact, mutually supporting, and, 

 with the modifications subsequently noted, they form 

 together a complex circumcellular organic skeleton. Some 

 walls are also infiltrated with mineral matters; espe- 

 cially are the outer walls of grasses and the like silicified. 



The form of the cell-wall is, of course, in all cases a per- 

 fect index of the form of the cell, although in some cases 

 after the death of the protoplasm the cell may be some- 

 what modified in shape. The wall is formed by the pro- 

 toplasm and it is properly regarded as a product of pro- 

 toplasmic metabolism or secretion. It is commonly 

 composed of two or three distinct layers, three often occur- 

 ring when the wall is strongly thickened. In special cases 

 where successive layers are deposited, the wall may present 

 a laminated structure. In the formation of the cell-wall 

 in general a middle lamella is first laid down. This is the 

 primary layer, and upon it is deposited a secondary, and 

 finally a tertiary, layer. The second layer is, as a rule, 

 the thickest or most completely developed. In the looser 

 tissues of the body the middle lamella may split at the 

 angles between the cells, thus leaving intercellular spaces 

 of greater or less extent, the importance of which in gas 

 diffusion through the plant will subsequently receive 

 special consideration. 



