464 BOTANICAL GAZETTE [june 



thin spots or pit areas and were more conspicuous in the winter than 

 in the spring and summer, a fact previously noted by De Bary (i). 

 Furthermore, Krxjger traced these structures to the "procambial 

 strands" and through the developing daughter cells of the cambium 

 to the highly differentiated cells of the xylem and phloem. They 

 appeared to be least modified in the walls of phloem parenchyma. 

 In the development of tracheids, vessels, and sieve tubes, owing to 

 increase in the size of the individual cells, they tended to become 

 more or less modified. KriJger concluded, however, that the thin 

 spots became enlarged and modified to form sieve plates (fig. 21), 

 and to form the primary pit areas of tracheary elements. He noted 

 that during this process the ridgelike thickenings tended to become 

 more or less modified in shape. Russow (11) and Strasburger 

 (15) emphasized the fact that in the Abieteae many of the pit areas 

 become more or less obliterated in the development of tracheids and 

 sieve tubes; that is to say, many of the primary pit areas become 

 vestigial, since they have no superimposed bordered pits or do not 

 become modified to form sieve plates. 



It is evident, therefore, that not only are bandhke thickenings 

 of the middle lamella of frequent occurrence in the tracheary 

 elements of Pteridophyta, Gymnospermae, and Angiospermae, but 

 also in the cells of the cambium and phloem. In discussing the 

 phylogenetic significance of the bandlike thickenings of the middle 

 lamella in tracheids of Coniferae, therefore, it is essential, not only 

 to compare these structures with similar structures which occur in 

 other types of cells and other groups of plants, but also to contrast 

 the various stages in their development. This must inevitably be 

 the case, since there is a considerable element of danger in basing 

 generalizations concerning relationships upon comparisons between 

 the structure of end products. Of course similar structures may 

 be attained through entirely different developmental stages. 

 Unfortunately, comparati^vely little is known in regard to the 

 detailed structure of the cambium and the various stages in the 

 development of tracheary pitting in different groups of vascular 

 plants. It is to be hoped that this gap will be filled in the near 

 future, since information of this character promises to throw con- 



