98 
most part very light and porous. Such an arrangement 
might have been expected in plants whose stems are only 
as thick as a finger, and whose sap has to travel a long di- 
stance before it can reach the leaves, which are for the most 
part met with only in the uppermost portions of the stems. 
In most of the species, this woody tissue is traversed by a 
large number of fine medullary rays, which give a beautiful 
figure to many of the sections. Their internal arrangement 
does not manifest itself in any marked way on their exterior ; 
their form is generally cylindrical, but some of them exhibit 
four slight projections in the form of narrow raised bands 
arranged lengthwise, which correspond with the outermost 
portions of the four cortical rays. Some species have a 
square stem during their early growth, and even the older 
stems do not altogether lose their four-sided character. 
The constancy of the figure four as the radical number 
is very noticeable in the structure of the different 
parts of these stems, and there can be little doubt that it 
originates in the decussate arrangement of the leaves. The 
stems of the Mints, Sages, and many other British plants 
furnish us with ready examples of a quadruple arrangement 
of parts. 
Mcdpighiacece. 
If the lianas which belong to the Bignoniacese are re- 
markable for the symmetry of their parts, the lianas of this 
family may be said to be characterized by an absence of 
symmetry. In general, their stems are singularly rugged 
in outline, a section presenting deep sinuosities or irregular 
projections, while at other times they appear to be made up 
of a number of separate branches which have become con- 
solidated in the progress of growth, so as to form a rough 
looking rope of many strands. 
Jussieu gives a> full and interesting account of the struc- 
ture of one of these stems, the Stigmaphyllon emarginatum 
