76 



FUNDAMENTALS OF CYTOLOGY 



Through the equator of the cell between the recently formed daughter 

 nuclei there is formed a continuous fluid film, the cell -plate, which extends 

 until it reaches the lateral walls. Physical and chemical changes, 

 including the deposition of pectin, transform this into a somewhat firmer 

 layer of intercellular substance, the middle lamella. The protoplasts 

 then deposit upon each side of the middle lamella a thin primary wall 

 of cellulose. This is the stage observed in the meristem. The primary 

 wall undergoes some thickening, but it remains very plastic during the 

 further growth and divisions of the cells. In some tissues no further 

 layers are added, the primary layers, with certain chemical transforma- 

 tions, becoming the walls of the mature tissue cells. 



In other tissues, notably woody ones, additional wall layers are added, 

 a secondary wall of cellulose being deposited upon each primary wall 

 (Fig. 54, h). Each secondary wall is composed characteristically of 



'..-.■ s 



lO 



Fig. 52. — Four stages in the establishment of the connection between the middle lamella 

 of a newly formed wall with that of the lateral wall of the divided cell, ml, middle lamella; 

 1, primary wall layer; cyt, cytoplasm. {After P. Martens.) 



three layers, of which the middle one is commonl.y the thickest. At this 

 stage the plasma membranes of the two protoplasts are separated by a 

 partition in which there can be distinguished as many as nine layers: 

 the two secondary walls each composed of three layers, the two primary 

 walls, and the middle lamella. This elaborate structure is not necessarilj^ 

 uniform over the entire extent of the wall, however. Here and there 

 are small areas in which no secondary layers are deposited, leaving pits 

 in which only a delicate membrane separates the two protoplasts. This 

 membrane may be pierced by fine pores, and in some cases it has a central 

 thickening, the torus. The secondary layer may overarch the margin 

 of the membrane, forming the bordered pit characteristic of certain 

 vascular cells of gymnosperms (Fig. 53). Another form of localized 

 deposition is seen in the spiral and ring-like thickenings formed by 

 protoxylem cells before their elongation has been completed. 



The manner in which the several layers of the newlj^ formed partition 

 become continuous with those of the lateral walls of the original meriste- 

 matic cell is illustrated in Fig. 52. As the extending cell plate, or young 

 middle lamella, meets the lateral partition, there is developed at its edge 

 a swelling with a minute cavity. This gradually extends through the 



