RESEARCH ON XYLEM AND PHLOEM IO3 



is, if one were small enough he could crawl into and out of the cells through 

 these openings — the original walls which were intact become clearly per- 

 forate. A series of these perforate cells arranged end to end forms a vessel, 

 the most efficient conducting element in xylem. Such multicellular structures 

 may extend, according to recent work, over the whole length of a tree trunk 

 and even into smaller and smaller branches, and probably into the roots as 

 well. This is an important factor in the possible spread of spores of some 

 pathogens in the plant, for example, and hence the development and spread 

 of a disease in an exceedingly short time. The number and shape of the 

 openings in the cells vary considerably throughout the secondary xylem of 

 dicotyledons. There may be a single large opening (fig. 5) in a transversely 

 placed end wall (the remains of which in many species occurs in the form of 

 a rim about the opening), or, at the other extreme, there may be a large 

 number of transversely placed elongate openings (fig. 2) on steeply inclined 

 end walls. Furthermore, the pitting on the side walls of these cells varies from 

 scalariform to opposite to alternate. The cross-sectional aspect of vessels 

 varies from rectangular to circular, their length from about as long as tracheids 

 to extremely short, their diameters from about comparable to those of 

 tracheids to large enough for the vessels to be seen with the unaided eye (con- 

 trast fig. 1, 2, 4, 5). Vessel members — the cells of a vessel — are extraordinarily 

 variable. 



For those interested in evolution, the fundamental question about these 

 upright cells in wood is: are they related to one another, and if so, in what 

 way? The two-pronged physiological definition of xylem as the principal 

 water-conducting and supporting tissue in vascular plants immediately sug- 

 gests that the earliest vascular plants had wood cells (tracheids) that served 

 both in conduction and in support. From these must have developed cells 

 (fibers) principally concerned with support and others (vessel members) 

 especially effective in conduction. To those particularly inclined toward 

 viewing the plant primarily as a functional mechanism, these "divisions of 

 labor" seem perfectly reasonable as evolutionary events. To the morpholo- 

 gist, too, such ideas are eminently sound, but he is likely to ask: is there 

 a way to clarify how and along what lines of an evolutionary nature such 

 changes took place? If a concept explaining the grosser aspects of the changes 

 could be developed, would its validity be supported by newly revealed infor- 

 mation? Put in another way, could the concept, or some variation of it, be 

 used in predictions? 



Jeffrey in this country and Boodle in England were early investigators of 

 the evolutionary development of vessels and other cells of the wood. Jeffrey 

 correctly concluded that vessel members arose from tracheids and that those 

 with scalariform perforation plates represent the beginning and that those 

 with simple plates are the end points in the evolution of vessel members. 

 But in supporting these ideas he used inconclusive evidence and, except for 



