1884.] Colonial Organisms. 247 
In the Endogen the colonial character of the tree is equally or 
perhaps yet more evident. The stem is, as it were, a fertile soil, 
in which annual colonies of leaf individuals take root, For the 
leaf is not properly an organ of the tree, but is an individual 
which is rooted in the tree, as the tree itself in the soil. The 
vascular bundles or the root fibers of the leaves force their way 
far down into the trunk, as if in search of nutriment, and thus 
add new tissue to the stem throughout its whole length. Neither 
in the Fern nor the Endogen has the stem any power of self- 
growth (except in some special instances), there being no ger- 
minal tissue except at the extremity of the stem. The tree fern 
adds new tissue mainly to the extremity of the stem, which in- 
creases in diameter only by the outgrowth of adventitious roots. 
The Endogen, by the downward growth of vascular bundles from 
the leaves, adds new tissue throughout the stem, which in conse- 
quence becomes exceedingly dense and hard, but which has no 
growth in diameter. 
In the Exogen other features of progression appear, and the 
colonial character has become less evident. The individual mem- 
ber of the Exogen colony is no longer a single leaf, but a group 
of leaves, seated on an unbranched connecting stem. And the 
vascular bundles proceeding from these leaves can be traced but 
a short distance into the substance of the stem. The members 
of the colony seek less for individual service by aid of their 
fibrous roots, but trust more to the general vascular service of the 
stem. This service is more easily performed from the fact that 
the exogenous trunk retains active tissue throughout its whole 
length. Thus cell reproduction is constant, and the stem in- 
‘creases annually in diameter. In addition the presence of ger- 
minal tissue throughout the stem permits asexual reproduction 
elsewhere than at the apex, as in Ferns and Endogens, and repro- 
Uctive buds may be formed at any point in the cambium layer. 
Thus the Exogen, though a colony in origin, has made a decided 
Movement towards a composite individuality, though it is far 
from having attained the definite individuality attained by the 
higher animal colonial groups. 
We have, thus far, considered only the leaf members of the 
Plant colony. There are two other members, the root and the 
flower, both produced by asexual budding, the former represent- 
ing an earlier stage in vegetable evolution than the leaf, the latter 
