CELLS AND TISSUES 7 



volume by expanding and increasing the surface area of the primary 

 wall, and are further characterized by their potentiality to undergo 

 reversible changes in thickness, etc.; Ql) the fibers, and other 

 highly specialized cells, which have lost the above potentialities 

 owing to irreversible changes; and, in addition 'to the primary 

 wall, which may be more or less specialized, have a supplementary 

 or secondary wall which is primarily mechanical in function and 

 incapable of growth or increase in surface area; and (3) the inter- 

 cellular layer referred to above as the intercellular substance or middle 

 lamella. 



Primary Wall Formation. — The first step in wall formation 

 is the development of the cell plate, which occurs in the equatorial 

 plane of the spindle during the later phases of nuclear division. 

 One theory of cell plate formation is that it is derived from the 

 materials of the spindle, and that the plate subsequently splits to 

 form two plasma membranes, which in turn secrete the pectic sub- 

 stances of the middle lamella. An alternative theory is that the 

 cell plate zone may be a more fluid layer resulting from the forma- 

 tion of two dissociated protoplasmic phases, one of which forms 

 the cell plate region and the other the plasma membranes which 

 secrete the pectic materials into the fluid layer. More investiga- 

 tion is needed on this point, but the concept of a fluid cell plate 

 zone is not out of harmony with the process of cell division by 

 furrowing such as occurs in the formation of the microspores of 

 many of the higher plants (Nicotiana, Medicago, Melilotus, 

 Lactuca, etc.) as well as in lower plant forms. In the microspore 

 mother cell, the furrows are initiated at the periphery and develop 

 centripetally in contrast to the formation of the cell plate from the 

 spindle, which is usually centrifugal. 



The primary wall, which consists of an anisotropic layer of 

 cellulose and pectic materials, keeps pace with the enlargement of 

 the protoplast by stretching and by the addition of new wall sub- 

 stances by intussusception or apposition. In the former, the addi- 

 tion of materials is carried on through a process of infiltration 

 in which additional substances are introduced among the older 

 ones; while in the latter there is a centripetal deposition of addi- 

 tional wall substance which forms thin plates or lamellae against 

 the original wall. It seems probable that in many instances, the 

 thickening of the wall is accomplished by the joint operation of 

 both processes. 



