﻿Brown: 
  Silicious 
  skeleton 
  411 
  

  

  w 
  

  

  1 
  

  

  easily 
  differentiated 
  from 
  each 
  other. 
  In 
  the 
  xylem 
  of 
  Tecoma 
  

   and 
  many 
  other 
  genera, 
  these 
  two 
  membranes 
  differ 
  little 
  from 
  one 
  

   another 
  in 
  appearance. 
  They 
  stain 
  nearly 
  alike, 
  have 
  nearly 
  

   the 
  same 
  index 
  of 
  refraction, 
  and 
  are 
  similar 
  in 
  chemical 
  and 
  - 
  

   physical 
  composition, 
  so 
  that, 
  without 
  special 
  technique, 
  they 
  

   are 
  indistinguishable 
  from 
  one 
  another 
  (as 
  shown 
  in 
  N, 
  Fig. 
  i). 
  

   They 
  may 
  be 
  differentiated, 
  however, 
  in 
  certain 
  refractive 
  media, 
  

   as 
  shown 
  by 
  the 
  writer 
  in 
  an 
  earlier 
  paper;* 
  or, 
  better, 
  by 
  mounting 
  

   cross-sections 
  10-20 
  ^ 
  thick 
  in 
  dilute 
  Shultze's 
  solution, 
  warming 
  

   the 
  slide 
  slightly, 
  and 
  observing 
  under 
  high 
  power. 
  In 
  Schultze's 
  

   solution 
  the 
  true 
  middle 
  lamella 
  is 
  dissovled 
  before 
  the 
  primary 
  

   thickening 
  of 
  the 
  fiber. 
  These 
  two 
  membranes 
  together 
  compose 
  

   about 
  one 
  half 
  of 
  the 
  cross-section 
  area, 
  and, 
  when 
  treated 
  

   with 
  phloroglucinol 
  and 
  hydrochloric 
  acid, 
  give 
  the 
  deep 
  red 
  

   reaction 
  for 
  furfurol. 
  It 
  is 
  also 
  a 
  noteworthy 
  fact 
  that 
  the 
  index 
  

   of 
  refraction 
  of 
  these 
  layers, 
  which 
  is 
  about 
  1.59 
  (that 
  of 
  the 
  middle 
  

   lamella 
  being 
  slightly 
  higher 
  than 
  that 
  of 
  the 
  primary 
  thickening), 
  

   approaches 
  the 
  refractive 
  index 
  of 
  the 
  calcified 
  wall 
  of 
  the 
  cal- 
  

   careous 
  alga 
  Penicilhis 
  {n 
  = 
  1.60). 
  Tecoma 
  forms 
  an 
  unusually 
  

   good 
  subject 
  for 
  study 
  because 
  there 
  are 
  few 
  genera 
  in 
  which 
  the 
  

   primary 
  thickening 
  is 
  as 
  well 
  developed 
  as 
  in 
  this 
  genus. 
  In 
  

   many 
  genera, 
  even 
  those 
  in 
  which 
  the 
  secondary 
  layer 
  is 
  well 
  

   developed, 
  as 
  in 
  Qtiercns, 
  Caesarea, 
  or 
  Aspidosperma, 
  the 
  primary 
  

   thickening 
  is 
  extremely 
  thin 
  and 
  may 
  even 
  appear 
  to 
  be 
  absent. 
  

  

  2. 
  Secondary 
  and 
  tertiary 
  thickenings 
  

  

  T 
  

  

  The 
  secondary 
  thickening 
  (5, 
  FiG, 
  i) 
  is 
  well 
  developed 
  in 
  all 
  

   the 
  lapachol-yielding 
  species 
  of 
  Tecoma, 
  and 
  appears 
  more 
  or 
  less 
  

   rounded 
  or 
  even 
  circular 
  in 
  outline 
  in 
  the 
  transverse 
  section. 
  The 
  

   index 
  of 
  refraction 
  of 
  this 
  thickening 
  of 
  the 
  fiber 
  is 
  slightly 
  below 
  

   that 
  of 
  quartz, 
  the 
  refractive 
  index 
  of 
  which 
  is 
  1.54. 
  A 
  thin 
  

   highly 
  refractive 
  tertiary 
  layer 
  (T) 
  borders 
  the 
  lumen 
  (L) 
  . 
  Neither 
  

   of 
  these 
  thickenings 
  reacts 
  for 
  furfurol, 
  and 
  both 
  are 
  therefore 
  

   unlignified 
  in 
  all 
  the 
  lapachol 
  species 
  of 
  Tecoma 
  examined. 
  In 
  

   other 
  genera 
  the 
  thickening 
  is 
  commonly 
  lignified. 
  

  

  When 
  cross-sections 
  are 
  first 
  mounted 
  in 
  glycerine 
  or 
  water, 
  

   the 
  secondary 
  layer 
  appears 
  to 
  be 
  of 
  homogeneous 
  composition; 
  

  

  * 
  Brown, 
  F. 
  B. 
  H. 
  The 
  refraction 
  of 
  light 
  in 
  plant 
  tissues. 
  Bull. 
  Torrey 
  Club 
  

   47: 
  243-260./. 
  1-4, 
  1920. 
  

  

  