42 BOTANICAL GAZETTE [july 



radiate from the clusters of protoxylem. These wide sheets of 

 homogeneous ray parenchyma do not arise by a process of 

 " compounding." Similarly, the " secondary" multiseriate rays 

 which develop as the root increases in circumference are not formed 

 by the " aggregation " and "fusion" of small rays, but originate, 

 as they do in the young stem and mature shoots, by the widening 

 of a single uniseriate ray. Conditions equally significant exist in 

 other genera of the Fagales. For example, wide multiseriate rays 

 occur in the first formed portions of the roots of Quercus velutina 

 Lam., although in the seedling stem only uniseriate rays are formed 

 by the first activity of the cambium. On the other hand, certain 

 species of the Betulaceae and Fagaceae possess only uniseriate rays 

 in the first formed portions of the roots, although so-called "aggre- 

 gate" or "compound" rays may develop during subsequent growth 

 (fig. 6). In the root, as in the seedling stem, variations in vigor 

 and other physiological conditions produce marked variations in 

 the development of rays. Vigor usually accelerates the formation 

 of large rays and feeble or stunted growth tends to reduce their 

 size or retard their development. 



THE REPRODUCTIVE AXIS 



Scott in his studies of cycads was the first to point out in a 

 convincing manner the importance of the anatomy of the reproduc- 

 tive axis as a possible seat of conservatism. More recently Jeffrey, 

 working upon the comparative anatomy of gymnosperms, has 

 reached the conclusion that this region is even more retentive of 

 ancestral characters than is the seedling stem. 



The flowering axes of many dicotyledonous plants are less 

 favorable for the study of structures which occur in the secondary 

 wood than are the cone axes of gymnosperms, since they often are 

 herbaceous or possess only a narrow zone of secondary tissue. It 

 is fortunate, therefore, that the peduncles of many species of Quercus 

 and C a sua r in a have wide zones of secondary wood and well devel- 

 oped rays. The peduncles of the red or black oaks (subgenus 

 Erytkrobalanus) are particularly interesting, since they persist 

 through two growing seasons and possess in consequence two 

 layers of secondary tissue. The development of ray structures 



