CHAP. II. 
EXOGENOUS STEMS. 
99 
tissue coated by a stratum of woody tissue, enclosing, at irregular 
distances from the centre, very unequal 37 
portions of the vascular system. The 
pith is exceedingly excentrical ; and the 
medullary rays, which are imperfectly 
formed, do not all radiate from the pith, 
but on the thickest side form curves 
passing from one side of the stem to the 
other, their concavities turned towards 
the pith. 
In the stem of a Bignonia in my 
possession, from Colombia {Jig. 38.), 
the vascular system is divided into four 
nearly equal parts, by four short 
thick plates radiating from the 
pith, and consisting of woody 
tissue, with a very few vessels. 
These plates are not more than 
one third the depth of the wood; 
so that between their back and 
the bark there is a considerable 
vacancy, by which the four di- 
visions of the vascular system are 
separated. This vacancy is nearly 
filled with bark, which projects 
into the cavity. 
In Staunton ia latifolia 
{fig. 39.), which has a 
twining stem, there are 
no concentric circles, and 
the medullary rays are 
curved, part from right 
to left, and part from left 
to right, diverging at one 
point and converging at 
another : the bark is pier- 
ced with extensive longi- 
tudinal perforations. 
H 2 
