ONTOGETSTY OF TRACHEABY ELEMENTS 85 



proportion of each t.vi)e varies within wide limits in different or- 

 gans and diff'erent plants. The assnmption is frequently made 

 that the significance of these various wall-patterns in protoxylem 

 tracheae is to permit the elements to "accommodate themselves" 

 to stretching. In the metaxylein or last-formed portion of the 

 primary xylcm, there is a marked increase in the relative extent 

 of secondary wall deposition, and elements with net-like thick- 

 enings {reticulate elements) and with pitted walls {pitted ele- 

 ments) are formed. A sharp transition does not exist, however, 

 between these varied wall-patterns and consequently the limits 

 between protoxylem and metaxylem can only be rather arbitrarily 

 established. Indeed, it is common to find tracheary elements with 

 several types of intergrading wall-patterns. Such transitional 

 types were early recognized and termed "vasa mixta." (Cf. 

 De Bar}', p. 156.) 



Apparently very little intensive study has been devoted to the 

 origin and mode of development of the fibrous type of secondary 

 wall thickening in primary xylem tracheae. Stover (1924) con- 

 tends that in Calamovilfa "the annular and spiral thickenings 

 are the direct result of elongation." He finds that the "first 

 thickening is laid down in the pitted form" and that "the division 

 and enlargement of the surrounding cells tears apart this wall 

 thickening and the cell becomes annular, spiral or reticulate, 

 depending upon the amount of stretching." This interesting 

 mechanical interpretation, however, could not be confirmed by 

 the work of Barkley (1927) on the differentiation of tracheary 

 elements in Trichosanthes. According to Barkley. the future pat- 

 tern of the secondary wall is determined early in ontogeny by a 

 peculiar distribution of vacuoles in the peripheral cytoplasm of 

 the procambial initial. She states that "the spiral vessel of the 

 protoxylem in its early stages has bands of peripheral cytoplasm 

 which precede the spiral markings and have the same arrange- 

 ment, and become the basis of the lignified spiral. The position 

 of the cytoplasmic bands is determined by rows of vacuoles in the 

 cytoplasm immediately preceding and during the formation of 

 the cytoplasmic bands." In a similar way, the annular and 

 reticulate types of thickenings are predetermined by the pattern 

 of vacuolation in the cytoplasm. It is evident, however, that a 



