78 



FIBERS 



If time permits, make similar studies and microehemical tests 

 of the bast fibers of various economically important textile plants, 

 such as "hemp" (Agave sp. and Ca7inabis sativa) and flax 

 (Linum u.sitatissimuni). 



2. Wood fibers. 



(a) Lihriform fibers. The fibers present in the secondary 

 xylem of woody dicotyledons often show massively thickened 

 secondary walls provided with scattered and rather small vestigial 

 pits. In such cells, which were termed "libriform" because of 

 their structural resemblance to phloem fibers, the lumen varies 

 in width and may be entirely occluded at certain points (cf. 

 Eames and MacDaniels, p. 63, Fig. 34, c, f, g). Study the form 

 and structure of the libriform fibers as shown in macerated wood 

 of oak (Qucrcns sp.). 



(&) Septate fibers. This type of wood fiber is characterized 

 by the subdivision of the lumen into a series of compartments 

 which are separated from each other by transverse walls or septa. 

 The work of Vestal and Vestal (1940) discussed in the Introduc- 

 tion of this exercise has shown that in Hypericum, the septa of 

 the fiber-tracheids arise, after the formation of the lateral second- 

 ary wall, as a result of repeated mitoses accompanied by cyto- 

 kinesis. Doubtless the septate fibers in other genera of the 

 angiosperms pursue a similar ontogeny. Secure a small amount 

 of macerated xylem from the stem of the grape-vine {Vitis sp.) 

 and study under high magnification the form and structure of 

 the numerous septate fibers. Note that the septa in these cells 

 extend to the inner edge of the secondary wall but are independent 

 of the compound middle lamella of the "mother cell." If the 

 septa are examined with the aid of an oil-immersion lens, it is 

 apparent that they have a laminated as well as a "pitted" struc- 

 ture. 



III. Suggested Drawings and Notes. — 



1. Prepare careful drawings to illustrate the form, character 

 of the lumeu and the type and distribution of pits in the bast 

 fiber of Tilid and the libriform fiber of Qucrcus. Label all im- 

 portant structures. 



2. Prepare a diagram to illustrate the arrangement of the 

 strands of bast fibers in the stem of Tilia. 



