ONTOGENY OF TRACHEARY ELEiVlENTS 8') 



(Bliss 1939). Save for these examples, and the Gnetales, vessel 

 elements seem to be restricted in their occurrence to the xylem 

 of angiosperms. The recent survey of Cheadle ( 1939 ) has brought 

 out the interesting fact that while vessels occur consistently in 

 the roots of monocotyledons, this type of tracheary element is 

 comparatively infrequent in the xylem of the stems and leaves 

 of this class of the angiosperms. The significance of this condi- 

 tion remains to be explained. In the dicotyledons, vessels appar- 

 ently are of widespread occurrence in both primary and second- 

 ary xylem, and have only been reported absent in Drimys, 

 Trochodcudron, Tetraccntron (cf. Bailey and Thompson 1918) 

 and certain members of the Crassulaeeae and Cactaceae. 



Two principal types of perforations occur in vessel elements, 

 viz.: (1) the simple perforation, which appears as a single large 

 oval or circular hole in each of the end-walls of the cell, a;nd 

 which is interpreted as the more advanced condition, and (2) 

 the sealariform perforation or perhaps more accurately the 

 scalariform perforation plate, which appears as a series of elon- 

 gated parallel openings separated by transverse bars, and which 

 is usually regarded as the more primitive type of perforation. 

 Simple perforations occur in vessel elements with either sloping 

 or transverse end-walls while scalariform perforations are typical 

 of elements with oblique or strongly-inclined end-walls (cf. 

 Eames and MacDaniels p. 65, Fig. 35). According to the in- 

 A-estigations of Cheadle (1939), in the majority of monocoty- 

 ledons examined the vessel elements possess the scalariform type 

 of perforation plate. A full discussion of the possible evolution- 

 ary history of the perforation-plate in vessel elements of the seed 

 plants is given by Jeffrey (pp. 92-102) and Bliss (1921). 

 2. Ontogeny of tracheary elements. 



(a) The formation of perforations in vessel elements. Re- 

 cent studies of this problem have centered about two important 

 questions, viz. : (1) the exact period or time in vessel differentia- 

 tion when the dissolution of the end-walls occurs, and (2) the 

 physical and chemical nature of the portion of the end-wall which 

 becomes perforated. According to Eames and IMacDaniels (p. 

 151), the perforation of the transverse end-walls of the vessel ele- 

 ments in the secondary xylem of the black locust {Rohinia 



