156 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



early period in the season, when water is plentiful, it may increase up 

 to four or five times the normal radial width of a cambial cell, reach- 

 ing a size of some 20 [i or more and equal to, or even greater than, 

 the tangential dimensions (Fig. 5A{b)). Increase in size is rather sudden, 

 occupying at most only a few hours, and involves the intake of relatively 



Fig. 54(6). Diagrammatic representation of two daughter cells in the cambium, 

 one of them (5) becoming differentiated as a tracheid (5'). The tracheid appears 

 here blunt ended; this is a consequence of the reduction of length in the drawing. 



large quantities of water. The increase in size is, in fact, due largely to 

 the development of a vacuole and the process is therefore called 

 vacuolation. All this time the wall surrounding the cell remains thin; 

 it is the primary wall whose structure and behaviour we shall examine 

 later on. Once the differentiating cell has reached its full size, how- 

 ever, the wall begins to thicken by the deposition of a series of 

 layers collectively called the secondary wall with which we are still 

 concerned, and these finally take the form which we have investigated 

 in the last chapter. 



