﻿The 
  silicious 
  skeleton 
  of 
  tracheids 
  and 
  fibers* 
  

  

  I 
  

  

  Forest 
  B. 
  H. 
  Brown 
  

  

  (with 
  five 
  text 
  figures) 
  

  

  I 
  

  

  CONTENTS 
  

  

  I. 
  INTRODUCTION 
  AND 
  DISCUSSION 
  OF 
  PROBLEM 
  407 
  

  

  II. 
  HISTOLOGICAL 
  ELEMENTS 
  OF 
  TRACHEIDS 
  AND 
  FIBERS 
  409 
  

  

  1. 
  Middle 
  lamella 
  and 
  primary 
  thickening 
  410 
  

  

  2. 
  Secondary 
  and 
  tertiary 
  thickenings 
  .' 
  411 
  

  

  3. 
  Histological 
  elements 
  of 
  the 
  secondary 
  thickening 
  412 
  

  

  a. 
  Silicious 
  skeleton 
  414 
  

  

  6. 
  Colloidal 
  matrix 
  419 
  

  

  III. 
  INHIBITING 
  EFFECT 
  OF 
  SILICIOUS 
  SKELETON 
  UPON 
  SWELL- 
  

  

  ING 
  AND 
  SHRINKING 
  OF 
  ELEMENTS 
  420 
  

  

  IV. 
  EFFECT 
  OF 
  DESILICIFICATION 
  UPON 
  THE 
  MECHANICAL 
  

  

  RIGIDITY 
  OF 
  THE 
  TISSUES 
  421 
  

  

  V. 
  SUMMARY 
  423 
  

  

  I. 
  INTRODUCTION 
  AND 
  DISCUSSION 
  OF 
  PROBLEM 
  

  

  It 
  is 
  a 
  remarkable 
  fact 
  that 
  when 
  the 
  water-saturated 
  secon- 
  

   dary 
  xylem 
  of 
  the 
  stem 
  is 
  allowed 
  to 
  dry, 
  it 
  shrinks 
  considerably 
  

   in 
  both 
  radial 
  and 
  tangential 
  diameters 
  but 
  only 
  slightly 
  in 
  length. 
  

   Conversely, 
  when 
  the 
  dry 
  secondary 
  xylem 
  imbibes 
  water 
  it 
  swells 
  

   mainly 
  in 
  its 
  radial 
  and 
  tangential 
  dimensions 
  but 
  remains 
  nearly 
  

   constant 
  in 
  length. 
  The 
  swelling 
  and 
  shrinking 
  of 
  the 
  xylem, 
  

   therefore, 
  takes 
  place 
  mainly 
  at 
  right 
  angles 
  to 
  the 
  axis 
  of 
  growth. 
  

   Relative 
  stability 
  of 
  length 
  during 
  the 
  imbibition 
  or 
  evaporation 
  

   of 
  water 
  is 
  a 
  strongly 
  maked 
  physical 
  property 
  of 
  the 
  xylem 
  in 
  

   either 
  the 
  living 
  or 
  dead 
  tissues 
  of 
  all 
  woody 
  plants; 
  and 
  the 
  bast, 
  

   in 
  all 
  cases 
  examined 
  by 
  the 
  writer, 
  has 
  been 
  found 
  to 
  exhibit 
  

   essentially 
  the 
  same 
  properties 
  of 
  swelling 
  and 
  shrinking 
  as 
  the 
  

   xylem. 
  

  

  The 
  amount 
  of 
  swelling 
  of 
  the 
  xylem 
  of 
  a 
  large 
  number 
  of 
  

   species 
  has 
  previously 
  been 
  determined 
  by 
  careful 
  measurements 
  

   without, 
  however, 
  arriving 
  at 
  a 
  satisfactory 
  explanation 
  of 
  the 
  

   phenomena 
  observed. 
  The 
  details 
  of 
  anatomy 
  of 
  these 
  tissues 
  

   have 
  not 
  hitherto 
  made 
  it 
  apparent 
  why 
  one 
  dimension 
  should 
  

  

  , 
  ^ 
  - 
  ■— 
  ■ 
  - 
  T.- 
  ^^ 
  

  

  * 
  Contribution 
  from 
  the 
  Osborn 
  Botanical 
  Laboratory. 
  

  

  407 
  

  

  