698 
Rhus  Venenata  DC. 
{Am.  Jour.  Pharm. 
Octobar,  1920. 
in  stems  of  the  Myricaceae.  On  treatment  with  alcohoHc  potash 
solution  the  resinous  material  combines  with  it  to  form  a  nigrescent 
compound.  This  reaction  is  characteristic^  of  the  poisonous  sub- 
stance causing  the  dermatitis  produced  by  poisonous  species  of 
Rhus  plants.  The  pith  consists  of  thin-walled  parenchyma  cells 
with  an  occasional  duct. 
HISTOLOGY  OF  LEJAF. 
The  lamina  is  of  dorsiventral  structure.  A  layer  of  thin- walled 
cells  forms  the  epidermis  of  the  ventral  surface.  In  the  mesophyll 
region  there  is  a  ventral  stratum  of  palisade  tissue  and  a  dorsal 
pneumatic  tissue  of  large  roundish  cells,  which  often  contain  rosette 
crystals  of  calcium  oxalate.  Stomata  lacking  subsidiary  cells  and 
raised  a  little  above  the  adjoining  epidermis  are  present  in  the  dorsal 
epidermal  region.  The  resin  canals,  which  were  present  in  the  fibro- 
vascular  bundles  of  the  stem,  also  accompany  the  vascular  bundles 
Fig.  2. — Transverse  section  through  midrib  and  portion  of  lamina  of  Rhus  vene- 
nata. Upper  epidermis  {ue) ;  palisade  parenchyma  {pal) ;  spongy  paren- 
chyma {sp) ;  lower  epidermis  {le) ;  resin  canals  {rc) ;  rosette  crystals  of  calcium 
oxalate  (c);  pericyclic  fibers  {pe);  hairs  {h). 
of  the  leaf  in  both  the  larger  and  smaller  veins.  Collenchyma  tissue 
is  present  in  the  dorsal  region  of  the  midrib.  Non-glandular  uni- 
cellular and  uniserate  hairs  are  found  in  the  dorsal  region.  They 
are  present  in  greater  number  along  the  veins.  Many  contain  the 
"resinous  substance"  of  Schwalbe,  which  responds  to  the  nigrescent 
reaction  when  treated  with  alcoholic  potash  solution.  Schwalbe^ 
accounts  for  the  presence  of  this  resinous  material  in  the  hairs  of 
Rhus  toxicodendron  as  a  product  of  osmosis  from  the  neighboring 
lacticiferous  vessels  containing  the  resin.  In  the  Rhus  venenata 
resin  canals  are  found  to  be  the  conducting  vessels.    As  the  resin 
