RESEARCH ON XYLEM AND PHLOEM 10 1 



thickenings (fig. 11) constitute the xylem elements formed while the stem 

 is still elongating as a whole — thus in accommodating over-all growth these 

 cells themselves may be stretched as the stem elongates. Such cells comprise 

 the protoxylem (fig. 9, PX). Those xylem cells subsequently formed as a 

 result of cellular activity in the stem tip comprise the metaxylem (fig. 9, 

 MX), whose mature cells are incapable of stretching because of the amount 

 and interconnected nature of their secondary-wall thickenings (fig. 12-14). 

 These two types of xylem together are called the primary xylem. In the 

 stems of many plants, only primary xylem is formed, and any increase in 

 the girth of such stems is due to increase in the size of the cells of other tis- 

 sues. This is true, for example, of most monocotyledons, some herbaceous 

 dicotyledons, and almost all ferns and club mosses. 



In the stems of other plants, including many woody herbs as well as 

 shrubs and trees, additional xylem is produced by the activities of a vascular 

 cambium, a generating zone of cells (fig. 9, VC) that originates between 

 the primary xylem and primary phloem. The girth of the stem in these plants 

 thus increases by the formation and enlargement of cells produced by this 

 cambium — secondary phloem (fig. 9, SPH) on the outer side and secondary 

 xylem (fig. 9, ESX and LSX) on the inner side. Most of the cells in these 

 secondary tissues are erect; that is, their long axes are parallel to the long 

 axis of the stem. There are, however, sheets of cells — usually living — that 

 are typically oriented at right angles to the elongate upright cells. These 

 radially disposed, thin sheets of cells are the rays [wood rays (fig. 9, WR) 

 or phloem rays (fig. 9, PR), depending on their location]. The vascular 

 cambium which produces these two categories of cells — one perpendicular, 

 the other horizontal — similarly has two types of cambial initials, one which 

 is short and produces the rays, the other which is long (fusiform in shape) 

 and produces all the vertical cells. A considerable body of data indicates 

 that the actual make-up of the cambium in terms of the ray and fusiform 

 initials is not static, but this information will not be considered further and 

 neither will wood nor phloem rays. 



We are now ready to consider the upright cells of the xylem, primarily 

 in terms of their evolutionary development. The investigations which led to 

 an understanding of the variations among these cells occurred within the 

 last fifty years, and chiefly within the last thirty. Some of the Americans 

 who were earlier involved in these studies are now gone, and some of them 

 have been proved wrong in their conclusions. Jeffrey, for example, and the 

 students who faithfully followed his pathways made mistakes, but E. C. 

 Jeffrey will always be remembered for his stimulating and aggressive stands; 

 he provoked the additional investigations that eventually were to correct 

 his mistakes. A sounder, more meticulous, but no less imaginative American 

 investigator took over from Jeffrey in developing the notions that now pre- 

 vail about evolutionary changes in the wood. That man is I. W. Bailey. 



