SIZE VARIATION IN TRACIIKAKY CELLS. 153 



types of organisms and under difTerent experimental or cnviromcntal 

 conditions. In certain tissues, in giv^en species or groups of plants or 

 animals, differences in growth or environment produce but slight 

 fluctuations from the norm or inherited cell size. This is not true, 

 however, for all types of cells under all conditions. 



That the extensive investigations of the comparative anatomist may 

 be of considerable assistance, as reconnaissance surveys, in the selec- 

 tion of suitable material for more intensive, experimental investiga- 

 tions, and to serve as checks upon too extensive generalization from 

 limited induction, is indicated by the literature dealing with body size 

 and cell size. For example, many difficulties might have been avoided 

 if the work of Levi (1906), upon the size of the various tissue-cells in 

 different groups of animals, particularly mammals, had preceded some 

 of the earlier, more intensive investigations. Therefore, in under- 

 taking a series of investigations upon the size variations of tracheary 

 cells, it seems to be desirable to begin with a reconnaissance survey 

 of cell size in the secondary xylem of vascular cryptogams, gymno- 

 sperms and angiosperms. 



In selecting this tissue for investigation the writers are influenced 

 by the following considerations: (1) Since the secondary xylem reaches 

 its optimum development in trees, lianas and large shrubs, plants 

 with a long reproductive cycle, it is a more favorable subject for the 

 extensive investigations of the comparative anatomist than the more 

 intensive cultural investigations of the geneticist. (2) From analogy 

 with animal tissues, it seems probable that the dimensions of the 

 highly differentiated tracheary cells of the secondary xylem may 

 fluctuate considerably in response to physiological and environmental 

 influences. (3) Inasmuch as this tissue is one of the most extensively 

 used botanical raw products, an investigation of this character may 

 throw some light upon a number of economic problems, particularly 

 those in the field of the paper-pulp industry. 



Material and Methods, 



Most of the material, used in the preparation of the following tables, 

 was secured from the extensive collections of the Bussey Institution 

 and Arnold Arboretum. Since the samples of wood and herbarium 

 specimens were collected at different times, and in different parts of 

 the world, by a number of taxonomists and dendrologists, it seems 

 to be advisable to tabulate them as far as possible under their original 



