88 TRACIIEARY ELEMENTS 



3. Maceruicd primary .rtjlein of ihc liypocotyl of hean ami 

 the ston of Trifolium. Ol)taiii small quantities of the macerated 

 tissue and study the form and wall-patterns of tlie various types 

 of isolated tracheary elements. Although many of the elements 

 may be broken or injured in the process of maceration, it is 

 possible to find intact tracheids or vessel elements. Note especially 

 the intergradations in certain elements between several different 

 types of secondary wall thickening. It is also instructive to 

 contrast the appearance of the rings and spiral bands in short 

 and elongated protoxylem elements. 



4. Macerated primary xyleyn of the rhizome of the bracken 

 fern {Ptericlium latiusculum) . Obtain a small amount of macer- 

 ated primary xylem of Pteridium, mount it in water and add a 

 cover-glass. The individual tracheary elements are large cells, 

 clearly visible to the naked eye, and vary in form from broadly- 

 fusiform or obovate to narroAvly-acuminate. Examine these cells 

 under low and high magnification, noting that one or both end- 

 walls are oblique with reference to the lateral walls. The latter 

 are provided with vertical series of typical scalariform bordered 

 pits. The presence of well-defined scalariform perforation plates 

 on the sloping end-walls of certain of these cells indicates that 

 they represent vessel elements. According to the recent work of 

 Bliss (1939, p. 620) "there are many cells that may be inter- 

 preted as transitional between the tracheid and the vessel ele- 

 ment." 



III. Material for the Study of Tracheary Elements in Sec- 

 ondary Xylem. — 



1. Tracheids of (jymuosperms. Obtain a small quantity of 

 macerated wood of Finns and study the form and pitting of the 

 tracheids. Three types of elements are present, viz. : (1) tracheids 

 from the "spring wood," characterized by their relatively wide 

 lumina and by the restriction of pits to the radial walls; (2) 

 tracheids from the "sununer wood," distinguished by their much 

 narrower lumiiui and by having the pits confined to the tangential 

 walls, and (3) fiber-tracheids which in their thickened walls and 

 reduced pits are intermediate in character between "typical" 

 fibers and tracheids. Note, especially in the spring tracheids, 

 that at certain points in the cell there occur groups of large, in- 



