THE PLANT BODY 43 



only, or do not exist. (These diflFerences have already been discussed 

 earlier in this chapter.) It has been reported that floral organs are 

 initiated in the inner tunica layers only, but carpel initials in some 

 taxa arise in part in the outer corpus, as do some leaf initials. In stamens, 

 marginal meristems are present in those that have laminar form and in 

 some of those with broad filaments. Ontogeny strongly supports, as 

 does vascular anatomy, the homology of all floral appendages with 

 leaves. 



Secondary Vascular Tissues and the Cambium 



Xylem. The xylem of angiosperms ranges widely in structure 

 from simple to highly complex and to a secondary simplicity de- 

 rived by reduction. The evolutionary story of the two cell systems in 

 xylem — vertical and transverse — and their cell types has been worked 

 out in detail. In the most primitive woods, the vertical system con- 

 sists of tracheids and a small amount of wood parenchijrrm; the trans- 

 verse system consists of parenchyma cells in the form of wood rays. 

 In specialization, the tracheids gave rise to vessel elements and various 

 types of fibers, and the wood parenchyma was increased in amount and 

 changed in distribution. In the transverse system, specialization changes 

 were in form, size, and structure of the rays. 



The primitive angiosperm tracheid is well shown in form and structure 

 by vesselless genera and those with very primitive vessels. This tracheid 

 is very long, with long, tapering, overlapping ends and numerous pits. 

 The pits of these tracheids are all scalariform — Trochodendron, Eupo- 

 matla. Such tracheids are remarkably fernlike. In other primitive genera, 

 some of the pits of the tracheids are round. Where both forms of 

 bordered pit are present, as in the Winteraceae, the scalariform type 

 occurs in the early-formed part of the annual ring. Scalariform pitting, 

 the undoubtedly primitive type of tracheary pitting in angiosperms, is 

 widely distributed among the less specialized families but is absent in 

 the higher families. 



The most primitive type of vessel in the angiosperms differs from the 

 primitive angiosperm tracheid only in the absence of closing membranes 

 in some of the scalariform pits of the end walls. In these vessel ele- 

 ments, the number of perforated pits is high — in Etipomatia, from twenty 

 to one hundred. They are arranged in ladderlike series along the 

 flattened, oblique end walls (Fig. 21A). In evolutionary modification, 

 this primitive vessel element became progressively shortened; accom- 

 panying the shortening, the advanced vessel element became rounded in 

 cross section, with the wall usually thinner than that of the primitive 

 element and often irregularly thick. The characteristic long-tapering 

 ends became less acute, until the end walls stood at right angles to the 



