204 PHARMACEUTICAL BOTANY 
In Gelsemium, species of Solanacea, Combretacea, Cucurbitacee, 
etc., there arises a cambium on the inner face of the xylem which 
forms tnternal phloem (or intraxylary 
| phloem), thus giving rise to bicol- 
| lateral bundles. (See Fig. 132.) 
In Strychnos Nux Vomica, 
internal phloem, exactly as in 
Gelsemium, etc. appears, but in 
addition interxylary phloem is 
developed. In the wood region 
of this plant axis the cambium 
starts at a certain age to lay down 
patches of phloem which become 
wedged in between xylem tissue 
as interxylary phloem. 
LENTICELS AND THEIR FoRMA- 
TION.—Lenticels are openings in 
the periderm which provide a 
Fic. 133.—White birch (Betula Means of communication between 
populifolia). Portion of a branch the external air and the living cells 
soil: the prominent _lenticels. of the bark beneath. The epi- 
oy dermis in a great majority of cases 
produces stomata, apertures, surrounded by a pair of guard cells, 
which function as passages for gases and watery vapor from and 
to the active cells of the cortex beneath. 
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P 
PLO ONS AR 3200 
PS 
Fic. 134.—Cross-section through a lenticel of Sambucus nigra. E, Epidermis; 
PH, phellogen; L, loosely disposed cells of the lenticel; PL, cambium of the lenticel; 
PS, phelloderm; C, cortical parenchyma containing chlorophyll. (From Sayre after 
Strasburger.) 
There very early originate in the region beneath some of the 
stomata loosely arranged cells called “complimentary cells which 
