I02 CHEADLE 



To I. W. Bailey we owe, among other accomplishments, not only the most 

 basic measuring device for evolutionary specialization of wood cells (cell 

 length), but also an understanding of the use and limitations of wood spe- 

 cialization in determining the phylogenetic relationships among woody plants. 

 His students and associates have developed, refined, and expanded his in- 

 vestigations to the point where the clarity of their conclusions stand un- 

 matched in the study of evolution in plants. It is about this subject that the 

 follo\ying paragraphs are written. 



If one were to examine the separated, vertically oriented cells of woods of 

 a great number of species of the angiosperms, he might readily be confused 

 by the bewildering array of sizes, shapes, markings, and structure of the 

 cells. Even in the nineteenth century, earlier anatomists had learned con- 

 siderable about the cells and had accurately described many of their fea- 

 tures, but it was not until the early years of the twentieth century that these 

 cells were studied from an evolutionary viewpoint. And not until Bailey and 

 Tupper published their notable study on length of elements in the secondary 

 xylem in relation to certain other features — including the length of the fusi- 

 form initials from which they were derived — did we have a reliable device to 

 use in any intelligent approach to the evolutionary problem represented by 

 cellular variation in the xylem. 



We must digress for a moment to review what variations there are in these 

 vertically elongate cells of the xylem. To reduce details, let us omit reference 

 to the typical living cells — wood parenchyma — important as they are, and 

 concentrate only on conducting and supporting cells of the secondary xylem 

 in the dicotyledons. These cells fall into three categories, one of which shows 

 especially conspicuous variations. The first of these categories consists of 

 tracheids (fig. 1 and others referred to in the legend for fig. 1). These are 

 elongate cells, relatively narrow in diameter, squarish to rectangular in cross- 

 sectional view, and with no openings from one cell to another. They are nor- 

 mally tapered gradually at either end and thus have no clearly defined end 

 walls. Their secondary walls are interrupted by bordered pits (fig. 7) (thin 

 places in the original primary wall overarched by the secondary wall). The 

 bordered pits are often round (fig. 6) or transversely elongate (fig. 12). The 

 transversely elongate pits (their occurrence in rows up the tracheid gives 

 the appearance of a ladder; hence pitting of this nature is called scalari- 

 form) are primitive features, as indeed is the tracheid itself. Other types 

 of pitting (fig. 13, 14) are considered more advanced. 



In the second category of cells are those which are even more slender and 

 thicker-walled than tracheids and which have no overarching borders on 

 their pits (fig. 8). These cells are fibers (fig. 3), and there are all gradations 

 between them and tracheids. 



The third category of cells are those which differ from both tracheids and 

 fibers in having actual openings (black in fig. 2, 4, 5) in their end walls; that 



