198 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
the present paper being devoted to the stem of Dicotyledons. The 
following are his general conclusions. 
The origin of the periderm may vary in the three following ways : 
it may be hypodermal, epidermal, or pericyclic. As a general rule it 
may be said to originate in the pericycle. It is both a protective tissue 
and a reservoir for food-materials. The central cylinder is always 
surrounded by a continuous ring of pericyclic fibres, and this ring is 
sometimes separated from the endoderm by a layer of cells, and the 
periderm then originates outside this layer. In the pericycle the peri- 
derm may either be in contact with the endoderm, or may be mingled 
with the fibres, or may spring from below the fibres ; it is always outside 
the liber, and therefore outside the outermost sieve-tubes. 
The position of the periderm is of but little value for purposes of 
classification ; it may be characteristic of an order, tribe, genus, or 
species. It is more developed in parts exposed to light than in the 
shade. The cortex disappears only to serve as food-material for the 
deeper tissues. The foldings on the radial walls of the cells, hitherto 
considered as characteristic of the endoderm, may belong to a secondary 
formation. 
Thickening-ring of Bark.^ — Herr M. Koeppen discusses the activity 
of the bark of our dicotyledonous trees during the period of activity of 
the thickening-ring. The passage from wood to bark is formed by a 
layer in which the new tissue-elements of both wood and bark arise. 
This is termed the thickening-ring, and consists of three zones : — the 
outermost comprises the young cells of the bark, the innermost the 
young wood-cells, while between them lies the true cambium. The chief 
purpose of the cortex is the conduction and storing-up of the substances 
which undergo metastasis in the green parts of the plant, though new 
substances are also formed in it. 
In the mode of growth of the secondary bark two types may be 
distinguished : — that of Tilia, in which the increase of girth is limited 
to the primary medullary rays, and that of Quercus, where the medullary 
rays usually consist permanently of one row of cells only, growth taking 
place in the rest of the parenchyme through the force of tangential 
traction. In the periderm are to be found cells which have more than 
doubled their length in the tangential direction without their walls 
having increased in thickness; and these cells always contain living 
protoplasm during the period of their increase in size. Beneath the 
epiderm there is often formed, for additional support, in the midst of the 
primary parenchyme, a ring of bast-cells and sclerenchyme-cells. 
Free Vascular Bundles in 01yra.f — In a large Brazilian grass be- 
longing to the genus Olyra Dr. F. Miiller finds that the cylindrical 
cavity of the hollow haulm is frequently occupied by spiral or twisted 
perfectly free vascular bundles, which frequently coalesce with one 
another or with the wall of the cylinder. Their number mostly varies 
between one and ten, though there are sometimes over twenty, and they 
are seldom found in all the internodes of the same stem, as some inter- 
nodes are usually entirely destitute of them. These free vascular bundles 
* Nova Acta Acad. Cses. Leop.-Carol., liii. (1889) pp. 441-96 (1 pi.). 
on r^ 4^.^ \ 
