30 FIRST FORMS OF VEGETATION. 
yellow, red, green, and brown, to the deepest and 
most sombre black. 
Though most of the peculiarities of mosses are 
visible to the naked eye, it is on the stage of the 
microscope that they appear to the greatest ad- 
vantage. The modifications of structure to suit 
the requirements of their economy thus revealed, 
cannot fail to excite our admiration.and astonish- 
ment. The stems of mosses, though serving the 
same purposes, are widely different from those of 
flowering plants.. We are ignorant of the manner 
in which they are developed. Probably, like en- 
dogenous plants, which is the least complicated of 
the two natural processes of increase in the vege- 
table kingdom, they grow by successive additions 
to the summit, never increasing the diameter after 
their outer layer has been formed, They are solid, 
and composed entirely of cellular tissue, which 
gradually becomes softer and more porous near 
the centre, uniform in every part, having neither 
medullary rays, nor true outward bark, nor central 
pith, nor even the scalariform vessels observable 
in the stems of ferns. Of the course taken by the 
ascending and descending sap, we are equally 
ignorant, if indeed there really exist in them 
currents similar to those of flowering plants, which 
may be more than doubted. The roots are ex- 
ceedingly delicate organs, and yet they take as 
