104 CHEADLE 



one unusual group of plants, even started with the wrong type of tracheid (in 

 terms of pitting). Thus a somewhat tortuous explanation emerged for some 

 of the variations we have cited. There was no real anchor, no conclusively 

 proved starting point, upon which his conclusions could be based. Yet he and 

 his students did provide a stimulus for the development of an evolutionary 

 point of view in studying xylem, a stimulus that eventually resulted in one 

 of the most completely documented illustrations of evolutionary changes in 

 plants. 



When Bailey and Tupper published their data in 1918 on the lengths of 

 various types of tracheids, fibers, and vessel members in the secondary xylem 

 of the stems of a wide variety of vascular plants and set these beside the 

 length of the cambial fusiform cells from which these elements were derived, 

 they provided a definite means for getting at the problem of evolution of 

 xylem elements. Once again we are struck with the importance of discovery 

 of a new approach to an old problem and how fruitful results of its use may 

 become in the hands of those who recognize its potentialities. Bailey's work 

 on the cambium itself showed that the fusiform initials are the origin of the 

 vertical elements of the xylem. Furthermore, it was shown that there is a 

 close relationship between the lengths of the initials and their derivatives. 

 Thus as the fusiform initials become shorter and shorter, so do their deriva- 

 tives in the xylem. In gymnosperms, the initials are extremely long; in the 

 flowering plants they vary from rather long to extremely short. One other 

 striking fact concerning the length relationships being discussed concerns the 

 possible elongation of the derivatives of the fusiform initials. When the initials 

 are extremely long, the derivatives increase in length very little. If the initials 

 are short, those derivatives which become vessel members increase little or not 

 at all and those destined to be tracheids increase somewhat, but those which 

 become fibers increase considerably in length by growth at their tips (in- 

 trusive growth). This complication does not in any way seriously affect the 

 use of length data, but merely makes it necessary to be wary in the over-all 

 study of wood evolution. 



With the recognition of a feature of wood that can be adequately meas- 

 ured, and therefore hallowed by statistical treatment, it was now possible 

 to gather the data available — all from present-day plants — and from these 

 provide a sound basis for theories on the evolution of vessels and other cells 

 in the xylem. P. H. Frost, working in Bailey's laboratory, made the first 

 definitive study about 1930 and then settled, for all unprejudiced anatomists, 

 the controversies concerning the details of vessel evolution, especially in the 

 secondary xylem of the dicotyledons. 



We do not intend to discuss here all the details of Frost's studies or of 

 those which preceded them and made them possible and worthwhile. A thor- 

 ough presentation would emphasize how a conclusive statement in this field 



