lo6 CHEADLE 



quence in the tracheid-vessel series which is correlated with a similar sequence 

 in the shortening of fusiform initials in the cambium which gives rise to 

 these cells. 



Is there a comparable series in the primary xylem of the dicotyledons 

 and of the monocotyledons? Later research by Bailey and Cheadle con- 

 clusively proved that the series is essentially the same in both these major 

 groups of plants, the only real difference being that the cells themselves 

 have greater average lengths and that there are greater technical difficulties 

 involved in making accurate observations. 



Let us see to what use some of the conclusions reached in the studies just 

 described have been put. As an example, it has been possible to determine 

 that vessels in the dicotyledons arose in woody plants (some present-day 

 trees and shrubs lack them) in the secondary xylem and then in succession 

 in the metaxylem (perhaps simultaneously in the last- formed metaxylem and 

 secondary xylem) and protoxylem, that subsequent specialization of vessels 

 took place in later-formed secondary xylem, early-formed secondary xylem, 

 metaxylem, and protoxylem, successively. (See fig. 9, LSX, ESX, MX, PX 

 for positional relationships.) So clearly established is this sequence that not a 

 single exception is known; those occurring in the literature have been shown 

 to be erroneous. 



As a second example, in the monocotyledons, vessels arose first in the 

 later-formed part of the metaxylem of roots and then appeared successively 

 upward in the plant. The same sequence is true of the specialization of ves- 

 sels. Furthermore, vessels arose and specialized in succession in the later- 

 formed metaxylem, early-formed metaxylem, and protoxylem of any particu- 

 lar organ of the plant in the monocotyledons. There are no reliably reported 

 exceptions to these statements. So clear is the situation in the monocotyledons 

 that given the information on vessels in the stem of a plant, one can predict 

 without exception the limitations of specialization within which vessel varia- 

 tion may be present in the remainder of the plant. In terms of evolutionary 

 studies, these sequences are fabulous illustrations of evolutionary change. 

 Furthermore, there are no identifiable evidences of retrogression; the se- 

 quences are unidirectional. 



As a third example, using the specialization of vessels as a base, it has been 

 possible to determine the evolutionary specialization of the arrangement of 

 vessels in the wood. For illustration, those woods in which the vessels are 

 distributed singly throughout mostly have primitive vessels. Conversely, when 

 the vessels are arranged in aggregations, the vessels are likely to be special- 

 ized. In those woods having large vessels in the first wood laid down after a 

 resting period (such as winter) and small vessels in later wood, the vessels 

 are chiefly specialized. 



Two other examples can be cited. The proper reading of the tracheid, 

 fiber-tracheid, fiber series can also be made by using correlation with speciali- 



