CHAP. II. 
EXOGENOUS STEMS. 
79 
In Stauntonia latifolia 
{jig. 37.), 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. 
3S In Euonymus tingens i^Jig. 
38.) the vessels near the centre 
of the stem are arranged in con- 
centric interrupted circles, but 
towards the bark there is no 
trace of such circles ; the sur- 
face of the stem is deeply cut 
into lobes parallel with the 
stem, and the vessels are all 
confounded in an uniform mass. 
Gaudichaud represents the stem of some Malpighiaceous 
plants to be in like manner divided into a number of regular 
lobes, which, however, actually reach the axis ; and, in con- 
sequence of the twining habit of the stem, are twisted into the 
appearance of a cable externally. 
In Menispermum laurifolium i^Jig. 
39.) the concentric lines evidently 
belong to the medullary system; they 
are extremely interrupted and un- 
equal, often only half encircling the 
stem, or even less, and they anasto- 
mose in various ways ; the medullary 
rays are unusually large, and lie across 
the wood like parallel bars ; and, 
finally, the plates of which the wood 
consists each contain but one vessel, 
which is situated at the external 
edge of the plate. 
