Peps ee 
RRS ge tee ee ee 
1878. ] On the Genealogy of Plants. 371 
The endogenous structure of monoctyledonous stems is of two 
classes. In the palms, as in the Cycadacee and arborescent ferns, 
the foliage springs from one terminal bud which attains its full 
development before expansion, after which no further lateral 
enlargement of the stem takes place. This may be regarded as 
the normal form of endogenous growth. But another form is 
found in the trunks of the arborescent Likiacee, as in Dracena, 
Yucca, Aloé, etc., which may be regarded as representing an 
advance in the direction of an exogenous structure. The stems 
of these tree-like Li/iacee actually undergo increase in size, or 
radial growth, after emerging from the bud. This takes place by 
the formation of a growing tissue (meristem) within the outer bark 
at certain distances below the terminal bud, which increases in 
thickness for some time before passing into permanent tissue, and 
effects an enlargement of the stem on all sides. A cross section 
of one of these trunks reveals a number of rings of this modified 
tissue, some of which are far internal, though at the time of their 
formation they must have formed the inner bark of the tree. 
Although this is clearly an advance towards the true exogenous 
structure, it seems to be rather by way of analogy than of direct 
progress, the same end (power of strengthening the trunk to 
resist the force of gravity and of the elements and thus to render 
greater size and longevity possible) being attained, but .by the 
adoption of a somewhat different means. 
There is another group of plants, wholly, different from those 
just described, which also afford considerable evidence of forming 
a transition stage from the endogenous to the exogenous struc- 
ture. These are the aquatic plants. Sanio observes that in Pota- 
mogeton and other aquatic and submersed Endogens, “an axial 
bundle extends continuously through the stem, the bundles from 
the leaves only subsequently uniting with it,” “a condition,” says 
Sachs, “quite anomalous in monocotyledonous plants, but also 
found in dicotyledonous water plants, particularly in the Vymphe- 
ace.” Such a condition found among aquatic plants is certainly 
very interesting in view of the probable aquatic character of all 
primordial vegetation, but whether these facts possess any real 
significance in connection with the question of the origin of the 
Dicotyledons still remains doubtful. 
In the venation of leaves of monocotyledonous plants, which _ 
is usually parallel, there are to be observed marked ie eer a 
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