AGE OF MONOCOTYLEDONS.—ACROGENOUS STEMS. 101 
rule may enable us to ascertain the age of a Palm, and probably 
also that of some other monocotyledonous trees, not the slightest 
dependence can be placed upon it in any particular instance, 
for there are frequently several rings produced on the stems of 
monocotyledonous plants in one year, and these again often 
disappear after having existed for a certain period. The best 
means of ascertaining the age of Palms is by noting their 
increase in height in any one year’s growth, and then, as such 
stems grow almost uniformly in successive years, by knowing 
their height we can determine their age. This mode, however, 
of calculating their age is very liable to error, and can be moreover 
but of limited application from the absence of data to work upon ; 
hence we must come to the conclusion that at present we possess 
no trustworthy means of determining the age of Monocotyle- 
dons. 
CO. AcrocENovus STEM, OR THE STEM OF CoRMOPHYTES.—The 
simplest form of stem presented by Cryptogamous plants is that 
seen in Liverworts (jig. 8), and in Mosses (figs. 9and 10). In 
such a stem we have no vessels, but the whole is composed of 
ordinary parenchyma, with occasionally a central cord of slightly 
elongated cells with somewhat thickened walls. In the stems of 
Club-mosses (Lycopodiacee) (fig. 12), Selaginellas (Sedaginellacee), 
Pepperworts (Marsileacex), and Horsetails (Hquisetacee) (fig. 13), 
we have the simplest forms of acrogenous stems, and the com- 
position of the fibro-vascular bundles, of which they are com- 
posed, and their mode of growth, have been already described 
(see page 78). The vessels found in the fibro-vascular bundles of 
the Lycopodiaceze are commonly spiral, and in those of the 
Equisetacez annular ; and as these bundles grow by additions to 
their apex, the stems of Cormophytes are termed acrogenous. 
In the Ferns (Filices) we have the acrogenous stem in the 
highest state of development. The Ferns of this country are 
comparatively but insignificant specimens of such plants, for in 
them the stem merely runs along the surface of the ground, or 
burrows beneath it, sending up its leaves, or fronds as they are 
commonly called, into the air, which die down yearly (fig. 14). 
In warm regions, and more especially in the tropics, we find such 
plants much more highly developed. Here the stem rises into 
the air to the height of sometimes as much as forty feet (jig. 15), 
and bears on its summit a tuft of fronds. In their general ap- 
pearance externally these Tree-ferns have great resemblance to 
monocotyledonous trees, not only in bearing their foliage like 
them at the summit, but also in producing no lateral branches, 
and being of uniform diameter from near their base to their apex. 
The outside of the stem of a Fern is marked with a number 
of scars, which have a more or less rhomboidal outline (jig. 
203). The surface of these scars presents little hardened 
projections, c, or darker-coloured spots, which appearance is 
produced by the rupture of some of the elements of the fibro- 
