46 BOTANICAL GAZETTE [july 



The stimulus of certain injuries may produce " aggregate" or 

 multiseriate rays in plants or portions of plants which normally 

 do not possess these structures. For example, when infected with 

 the chestnut bark disease, Endothia parasitica Murrill, the chestnut 

 may form large, completely parenchymatous sheets of ray tissue 

 (fig. 1 6), just as " aggregate" or false rays may be recalled in the 

 stems and roots of Betulaceae which normally possess only uni- 

 seriate rays (fig. 12). Wide multiseriate rays may also be induced 

 in certain of the Betulaceae by traumatic stimuli. Especially 

 significant are the effects of injuries in seedlings, roots, and the first 

 formed portions of the mature shoots. Injured seedlings of 

 Quercus velutina show an abrupt transition from congeries of uni- 

 seriate rays to wide sheets of homogeneous ray parenchyma. 

 Furthermore, a seedling stem or root of Quercus alba, which pos- 

 sesses only uniseriate rays, will, when injured, form wide multi- 

 seriate rays (fig. 21). These rays often develop without indication 

 of the putative stages of " compounding " (fig. 6). Similar phe- 

 nomena occur in the first formed portion of mature shoots, in those 

 radii of the stem which do not normally possess wide rays (fig. 10). 



Discussion of evidence from conservative regions 



In applying Haeckel's doctrine of recapitulation, the fact that 

 plants develop by the activity of growing points or meristems 

 rather than by interstitial growth has not always received sufficient 

 consideration. Among higher animals the embryonic and very plas- 

 tic stages of development are replaced by more highly organized 

 and stereotyped conditions during early stages of ontogeny. In 

 plants, however, the undifferentiated meristematic tissues retain 

 much of their primitive plasticity throughout the life of the indi- 

 vidual. This is shown by the fact that the growing points or 

 meristems of mature plants are potentially able to reproduce the 

 whole organism. Owing to this fact, that embryonic types of tis- 

 sue are active throughout the life history of plants, phenomena of 

 recapitulation need not be confined necessarily to the so-called 

 seedling plant, and may be expected to be more varied than 

 in the higher animals. Thus, there appears to be no fundamental 

 a priori objection to the supposed natural conservatism of the first 



