FIBROVASCULAR TISSUES: RAYS 65 



in the Paleozoic, that rays in this type were more or less largely 

 composed of tracheids. It seems clear for this reason that the 

 radial bands of storage parenchyma which constitute the rays 

 in secondary wood, like the longitudinal parenchyma included 

 under the caption of wood parenchyma, are derived from the 

 modification of tracheary tissue. It thus becomes apparent that 

 originally all the parenchymatous constituents of wood, whether 

 primary or secondary or radial or longitudinal, in the first instance 

 made their appearance by the modification of tracheary tissues. 

 Tracheids, in fact, constitute the sole original feature of organiza- 

 tion in woods, and the course of evolution expressing itself in con- 

 tinued differentiation has led to the derivation of all the other 

 features of ligneous structure from this primary constituent. In 

 other words, wood primitively was a purely water-conducting 

 tissue, and the superadded mechanical and storage functions 

 subserved by its organization in later geologic times resulted in 

 appropriate modifications of the original tracheary elements. The 

 derivation of the rays from tracheary tissues can be distinguished 

 clearly only in the lepidodendrids and their allies. In others of the 

 arboreal cryptogams which were so characteristic of the forests of 

 the Paleozoic age no evidence of the origin of ray cells from tracheids 

 has been observed. The same statement holds for the lower and 

 ancient gymnosperms, the Cycadofilicales (Pteridospermae of Oliver 

 and Scott) and the Cordaitales and their allies. These antique 

 gymnospermous groups, as well as the arboreal cryptogams sig- 

 nalized above, had no storage tissues in their wood other than 

 radial parenchyma ; for the longitudinal parenchymatous elements 

 known as wood parenchyma proper made their appearance only in 

 connection with the seasonal refrigeration which became ever more 

 pronounced during Mesozoic and later geologic time. The rays 

 of the older plants with secondary growth were of two main types. 

 In some instances (e.g., Cycadofilicales) they were composed of 

 bands of cells several elements in width and greatly varying in 

 height. In the Cordaitales and allied forms the rays were ordi- 

 narily uniseriate that is, a single cell in width, in contrast to their 

 often multiseriate and considerable height. In a general way it 

 seems clear that woods of the first type are perpetuated in the still 

 living, although much reduced, Cycadales, while the Cordaitales, 



