Meristems 



83 



long, grow not only by cell stretching as the internode elongates but 

 also by localized growth at both tips, as a result of which the fibers push 

 in between adjacent ones. In the readjustments thus made necessary 

 it is essential that where pits are present the pit fields in adjacent cells 

 develop opposite each other, since there must be a corresponding opening 

 in each wall. This evidently takes place after the relative position of the 

 walls has become fixed. The problem of "sliding growth" has been dis- 

 cussed at length bv Meeuse (1942). 



The increasing circumference of the axis also requires that the number 

 of rays be continually increased if the proportion between rays and verti- 



¥ 



Fig. 4-21. Radial section of wood of Chamaecyparis, showing transitional cell types 

 associated with the origin of a ray from a fusiform cell. ( From Bannan. ) 



cal elements is to be maintained. The origin of new rays has been de- 

 scribed for gymnosperms by Bannan ( 1934 and Figs. 4-20 and 4-21 ) and 

 by Barghoorn (1940a and Fig. 4-22) and for certain angiosperms by 

 Barghoorn (1940b and Fig. 4-23). A new ray arises from a short cell cut 

 out of the radial face of a fusiform initial, the nucleus first migrating to 

 the particular place where the new daughter cell is to be produced. The 

 height of the ray is then increased by transverse divisions of this cell and 

 its products, and its width by radial divisions. High rays may break up 

 into shorter ones. The rays as seen in tangential section maintain an al- 

 most constant distance from one another. How this is accomplished is 

 described by Bannan (1951). 



