CONSTANCE: SYSTEMATICS OF THE ANGIOSPERMS 415 



phylogeny of angiosperms may be of use even though this reviewer's interpreta- 

 tions may not be accepted. I am only too conscious of the fact that many im- 

 portant omissions have been made, but I have been as thorough as time and 

 available resources permitted. 



Wood Anatomy 



The most firmly established series of unidirectional phylogenetic trends 

 witliin the angiosperms are undoubtedly those having to do with features of the 

 secondary xylem. Bailey (1949, 1953) has indicated that the possession of tra- 

 cheary tissue is a significant mark of land plants at least as far back as the 

 Devonian. He emphasizes the dual role of this tracheary tissue in conduction 

 and in affording mechanical support, and shows how efficiency of transport has 

 been obtained through changes of form and loss of protoplasmic contents by 

 tracheary elements, culminating in the development and specialization of ves- 

 sels, and provision of mechanical strength through the physical structure and 

 chemical constitution of the cell walls. 



The major trends of phylogenetic modification of the tracheary tissue of the Land 

 Plants are associated with changes of equilibrium between these two fundamentally 

 important physiological functions. . . . These salient trends of evolutionary specialization 

 of the tracheary tissues are largely unidirectional and irreversible, and are fully pre- 

 served in surviving angiosperms. There fortunately are no serious missing links in 

 these phylogenetic chains and it is not essential, for example, to search geological strata 

 for vesselless proangiosperms since ancestral types of primitive xylem occur in living 

 representatives of both the dicotyledons and the monocotyledons (Bailey, 1949, p. 66). 



The classic work of Bailey and Tupper (1918) in measuring tracheary cells 

 from a very broad range of plant groups represents, in the words of Metcalfe 

 and Chalk, "the beginning of a new period of phylogenetic wood anatomy" 

 (1950, l:xlii). Bailey and Tupper found that the length of cambial cells and 

 their derivatives is progressively reduced in advancing series of vascular plants. 

 Basing his work on this fundamental principle. Frost (1930a, 1930b, 1931) estab- 

 lished a sequence of vessel types in angiosperms. He reasoned that, if a vessel 

 segment is derived from a transformed tracheid, then a primitive vessel should 

 have a maximum of tracheid-like characters, viz., great length, small diameter, 

 angular outline, uniformly thin walls, and very slight development of end walls. 

 By statistical methods he was able to show that such vessel types are correlated 

 with exclusively scalariform perforations. Frost then constructed a phyloge- 

 netic sequence of types of vessel pitting, beginning with scalariform and culmi- 

 nating with transverse-porous. Kribs (1935) employed this sequence of vessel 

 types as a basis for an attempted development of a phylogenetic sequence of 

 wood rays. He found that, statistically, heterogeneous rays tend to be associated 

 with wood containing scalariform vessel elements and homogeneous rays with 

 porous vessel elements. He was able on this basis to classify all wood rays into 

 six groups of varying grades of evolutionary specialization. The subsequent 

 studies by Barghoorn (1940, 1941a, 1941b), introduced ontogenetic data and 

 some needed cautions against the tendency to oversimplify ray classification. 

 According to Metcalfe, "The investigation of phylogenetic trends on the basis of 

 ray structure is not very satisfactory, however, because the different classes of 

 ray are not very well defined" (1946, p. 168). A correlation of type of lenticel 



