106 
ORGANOGRAPHY. 
BOOK I. 
from a figure of Meneghini (liicerche sulla Struttura del Caule 
nelle Piante Monocotyledon^ t. ix. f. 1. c.), to arrange its woody 
bundles in concentric layers when old; Smilax has a stem 
strictly endogenous, and a root which approaches in structure 
the stem (not root) of Exogens ; and, in the article Endo- 
gens, in the Penny Cyclopaedia, I have shown that the stem 
of Barbacenia is composed of roots of an endogenous nature, 
held together by the adhesion of their cortical integument, 
and that of a very slender central true stem on which they 
are moulded. In Grasses the stem is hollow except at the 
nodes, where transverse partitions intercept the cavity, divid- 
ing it into many cells. In the Bamboo these cells and parti- 
tions are so large that, as is well known, lengths of that plant 
are used as cases to contain papers. But if the gradual 
developement of a grass be attentively observed, it will be 
found that the stem is originally solid ; that it becomes hollow 
in consequence of its increasing in diameter more rapidly 
than new tissue can be formed ; so that its deviation from 
the ordinary characters of Endogenous structure is much less 
considerable than it seems to be at first sight. 
According to Mohl, the structure of an Exogenous and an 
Endogenous stem, during the first year of their growth, is al- 
together the same ; but in the second year the wood and the 
liber of the former separate, and new matter is then inter- 
posed, while, on the contrary, in Endogens no such separa- 
tion occurs, and consequently the newly-formed matter of the 
stem is fofced towards the centre, through which it passes, 
with a constant tendency, however, to reach the outside. I 
confess, however, I do not perceive this analogy ; on the con- 
trary, if we compare the new shoot of an Asparagus and that 
of an Elder-bush, the difference between them will be too 
great to be thus explained away. M. Dutrochet thinks that 
in the globular rhizoma of Tamus an argument may be found 
to show the identity of Exogens and Endogens in the first 
period of their growth; and this may, perhaps, be admitted; 
but it is equally evident, from the same example, that they 
become entirely different immediately after the first period. 
Nor, indeed, is the anatomy of the woody tissue, which con- 
stitutes the ligneous wedges of Exogens, the same as that 
