1914] BAILEY &■ SINNOTT—PHYLOGENY OF ANGIOSPERMS 55 



tion of their phloem extensions (figs. 1, 2, 7, etc.). It is significant, 

 therefore, that these typical intruding wedges of sclerenchyma 

 occur in the Betulaceae opposite each " false" ray of the xylem 

 cylinder (fig. 12). 



C asuarinaceae . — Successive stages of the reduction and dis- 

 integration of wide multiseriate rays, such as are found in the 

 genus Quercus, occur in the shea-oaks or Casuarinas. There are 

 several species in this family, however, in which the wide multi- 

 seriate rays are persistent in the first formed portions of the stem . 

 In the subsequent development of these wide rays reduction and 

 disintegration occur. These dissected sheets of smaller ray tissue 

 become diffused through certain radii of the stem, and in this way 

 wide multiseriate rays are replaced by narrower biseriate or tri- 

 seriate ones. At the same time, certain of the uniseriate rays which 

 occur between the wide rays widen until they become as broad as 

 the narrow sheets of tissue which are dissected from the original 

 wide ray. A somewhat similar phenomenon appears to have 

 occurred in the genus Betula, in which narrow biseriate or triseriate 

 rays have replaced the original wide multiseriate rays. As in the 

 r agales, wide multiseriate rays may be recalled in the Casuarinaceae 

 by stimulating growth or injuries, and tend to be persistent in the 

 peduncle and the vicinity of the traces of the leaves and rootlets. 



Origin, distribution, and antiquity of multiseriate rays 



Multiseriate rays of varying width are well developed in the 

 majority of arborescent or shrubby dicotyledons and may be 

 traced through the Tertiary to the Middle Cretaceous. These 



ma 



habitat. Very wide multiseriate rays occur in presum 



more temperate 



mangroves, desert plants, semi 



shrubs, etc., which are adapted to peculiar environments. The 

 investigation of the structure and development of rays in the 

 various families of the dicotyledons reveals much evidence that the 

 multiseriate rays originated by the gradual widening of primitive 

 uniseriate rays. Detailed evidence in favor of this view will be 



in 



