MATERIAL FOR STUDY OF FIBERS 77 



advantageous when the very considerable length of many bast-cells 

 and their active growth in length and thickness are taken into 

 account." In certain types of w'ood fibers, however, mitosis is 

 followed by cytokinesis, resulting in a chambered or septate fiber. 

 This condition has been observed and described by Vestal and 

 Vestal (19-40) in a recent study of the septate fiber-tracheids 

 of Hypericum Androsaeminn. In this species, the fiber-tracheid 

 retains its protoplast after the thick secondary wall has been 

 laid down. Mitosis may then occur in such a cell, the division 

 figure being oriented parallel to the long axis of the cell. Cell 

 plate formation then occurs in the normal manner and a thin 

 transverse septum is formed across the lumen, intersecting the 

 inner edge of the secondary wall of the "mother cell." Because 

 of the delicacy of this septum it Avas not possible to determine 

 whether it is "formed only of intercellular cement substance 

 or whether it consists of the intercellular substance and two 

 adjacent primary walls." 



II. Material for the Study of Fibers. — 



1. Bast fibers. Examine, under low and high magnification, 

 macerated bark of the twig of the basswood or linden tree (Tilia 

 sp.). The numerous prosenchymatous cells present are bast fibers 

 from the phloem region. Select an unbroken fiber and study 

 carefully its form and wall structure. Note especially the 

 channel-like lumen and the small vestigial pits. According to 

 Eames and MacDaniels (p. 57), the pits in bast fibers represent 

 modified simple pits. To appreciate fully the arrangement and 

 mechanical significance of the fibers in the phloem of Tilia, strip 

 off a small portion of the bark from a twig or young branch and 

 scrape off the outer tissues (i.e. epidermis, periderm and cortex) 

 with a scalpel. Then mount the fibrous tissue which has been 

 exposed in water and examine it under low magnification, noting 

 the closely-joined strands of grayish-white bast fibers. In order 

 to determine the degree of lignifieation of the secondary walls, 

 treat separate portions of the fibrous network with (1) IKI and 

 sulphuric acid,^ and (2) phloroglucinol and hydrochloric acid.- 



1 Cf. Appendix, p. 141. 



2 Cf. Appendix, p. 141. 



