RESEARCH ON XYLEM AND PHLOEM IO7 



zation of vessels in the same woods. The evolutionary specialization in pat- 

 terns of wood parenchyma have also been more or less clearly established 

 by utilizing variations among the accompanying vessels as a measuring device. 



While the major evolutionary tendencies within secondary xylem, of which 

 the above are examples, can thus be established, it should be carefully noted 

 that it is principally the different rates of specialization of these features in 

 any given wood that provide the important methods of microscopically dis- 

 tinguishing one wood from another. Much of this type of work, incidentally, 

 has been done by researchers in other countries. 



There is another aspect of the use of specialization of vessels, and of xylem 

 generally, that should be mentioned. If it is accepted that specialization in 

 vessels is a unidirectional evolutionary sequence, then this sequence is of 

 immense value as a means of determining the phylogenetic placement of 

 plants in the schemes of natural classification. For example, it is clear that 

 typical herbs in the dicotyledons must have arisen from woody ancestors, 

 for, on the one hand, herbs generally have extremely specialized vessels and, 

 on the other, there are some woody species in the dicotyledons that lack ves- 

 sels of any kind. As another example, the vessel situation in herbaceous mono- 

 cotyledons is such that they could not have originated from the dicotyle- 

 donous herbs suggested as their forebears by some writers. Like any other 

 feature concerning plants, the tracheid-vessel series must be intelligently 

 employed. But this series has the obvious advantage of being the most clearly 

 documented of any that might be used. Because of its unidirectional develop- 

 ment, it can always be employed as a negating factor; for example, a plant 

 with highly specialized vessels in the secondary xylem could not have given 

 rise to one with only normal tracheids in secondary xylem. The application 

 of the tracheid-vessel series in this sense is its most compelling attraction as 

 a tool for use in developing a natural classification. 



If there are those who believe that xylem has been pretty well worked 

 over, even from an evolutionary standpoint, let it be said at once that there 

 are problems both large and small still to be solved. If we are to malce the 

 best use of vessels in classification, there are enormous numbers of plants 

 still to be examined. It is especially necessary further to investigate her- 

 baceous, aquatic, and parasitic or saprophytic dicotyledons. The same is 

 true of a wide variety of monocotyledons, not so much to test further the 

 generalities that have been made in the last few years as to establish a more 

 complete inventory for use in phylogenetic classifications or in interpretation 

 of the loss of xylem in certain aquatics and other plants in which typical 

 xylem is lacking. Without such investigations, we lose full use of an effective 

 tool in evolutionary studies. 



Before closing this brief account of some aspects of progress in research 

 on xylem, I should make clear that there have been many other extensive 

 investigations on this conducting and supporting tissue. In particular, we 



