54 MORPHOLOGY OF THE ANGIOSPERMS 



Ray systems commonly show no definite pattern or uniformity of 

 arrangement. Nodal regions may be exceptions where primary rays are 

 related to leaf traces and gaps and, in some highly specialized woods, 

 where the rays, of fairly uniform size and shape, lying in transverse 

 tiers, are storied. Storied rays occur in scattered families: Bignoniaceae, 

 Compositae, Leguminosae, Meliaceae, Tiliaceae, Ulmaceae, and a few 

 others. They may characterize an entire family or only part of the 

 genera. 



The early history of the ray in vascular plants is unknown. Multi- 

 seriate form goes back to Devonian taxa. In angiosperms, the primitive 

 ray system consists of both uniseriate and multiseriate types, extending 

 from the margin of the primary xylem. The multiseriate are heteroge- 

 neous, and the uniseriate, high celled. Although rays frequently increase 

 in size as they elongate, there seems to be no evidence that, phylo- 

 genetically, the multiseriate ray is an enlarged uniseriate ray. Change 

 in size and form of the ray as secondary growth continues is common 

 in some taxa, rare or absent in others. 



Phylogenetic specialization of the ray system seems to be by simpli- 

 fication; a system made up of only one type of ray is developed by 

 suppression of the other type. In this way, systems consisting of only 

 uniseriate or only multiseriate rays have been developed. In multi- 

 seriate rays, the trend in modification may be from homogeneous to 

 heterogeneous or vice versa. Within the angiosperms, a system con- 

 sisting wholly of uniseriate rays seems to be a highly specialized type, 

 which has developed independently in a few taxa — Salicales, Sapindales, 

 Castanea. 



Rayless secondary xylem doubtless represents the greatest modifica- 

 tion in the transverse conducting system. No rays are present in the 

 secondary wood of many herbs, even among those with well-developed 

 woody cylinders. Little is known in detail of the histology of the wood 

 of herbs and subshrubs, but it is apparent that, in these plants, rays 

 may be lost by ti-ansformation into wood parenchyma. (The rays, usu- 

 ally of upright cells, become merged with surrounding wood paren- 

 chyma.) But rays are apparently lost also by dropping out, onto- 

 genetically or phylogenetically; some herbs that have secondary wood 

 made up chiefly of fibers are rayless. Rayless secondary wood occurs in 

 woody herbs and subshrubs in widely scattered families — Geraniaceae, 

 Tremandraceae, Crassulaceae, Empetraceae, Caprifoliaceae. 



The loss of wood rays in herbs is associated with reduction in cambial 

 activity and the consequent drop in demand for radial conduction. 

 Histologically, distal termination of rays comes about by transformation 

 in the cambium of ray initials into fusiform initials. As the axis increases 



