STEMS, LEAVES, AND BUDS. 
39 
a Avoody substance, as exhibited in tig. 3. This filling up sub- 
stance, on making a transA^erse incision, exhibited rather irre- 
gular layers, Avith a moderate magnifying poA^er. It appeared 
as seen in fig. 9 ; and on being magnified by 315 diameters, it 
evidently consisted of strata of larger and smaller cells, partly 
filled up, partly empty, Avith interstices, as seen in fig. 10. The 
circumstance, hoAvever, Avhich appears particularly remarkable, 
is, that the internal structure of the filling up substance, as 
exhibited in fig. 11, on a longitudinal incision, corresponded 
A^ery nearly Avith the old v^ood, fig. 12, situated next to it, 
Avith the dilference only, that spiroids existed in the former, 
Avliich Avere entirely absent in the nev^ v^ood. It Avill be seen, 
therefore, that the formation of layers is peculiar to the Avood, 
and is by no means caused by any external influences. 
A treatise of D. Don, on the Cycadem (Annals of Natural 
History, vol. v. p. 48), Avas read at a Meeting of the Linnsean 
Society, in January, 1840. The author states, the stem in 
the Conifer ce exhibits the formation of other Dicotyledons ; 
the annual zones are distinctly marked, and a distinct sepa- 
ration is also seen betAveen wood and bark ; this is not the 
case AAlth the Oycadece. They resemble the Monocotyledons 
in this respect, but are distinguished from them in others. 
Cycas has, besides a large central pith, several thick, concen- 
tric, developing layers of cellular and fibro-vascular tissue ; 
in Zamia and Enceplialartos, there are, besides the pith, only 
two very thick layers, an internal layer of fibrous tissue, and 
an external layer of cellular tissue. The Cycadece do not 
share the peculiarity of the wood of Coniferce, which consists 
of parallel regular dotted tubes ; but there are, in it, dotted, 
reticular, and spiral vessels, besides slender transparent tubes, 
which are without dots or any other distinction. The dotted 
vessels of the Cycadece certainly resemble the same tissue of 
the Coniferce, but the dots are placed much more irregularly, 
not only in different vessels of the same bundle, but also in 
different localities of the same Axssel. Their form is oblong 
or elliptical in Cycas revoluta, glauca, and speciosa ; Zamia 
furfur acea and pumila ; as also in Encephalartos horridus 
and spiralis; but sometimes they are longer, smaller, and 
431 
