ANATOMY OF TISSUES. 99 
manner of growth till the fifth or sixth leaf is formed; then 
with a sudden enlargement of the diameter of the stem, another 
method of arrangement ensues which may be described as a 
widening out of the axillary bundle into a hollow cylinder, 
so that there is a central pith and an outer rind. Since the 
parenchymatic cells of the pith act partly as conducting cells, 
and partly as reserve cells for the food material of the plant, 
it would be very disadvantageous to have them entirely isolated 
from the parenchymatic cells of the rind by a 
continuous zone of vascular bundles. <A con- 
nection is therefore secured between the pith 
and rind by means of what is known as the leaf 
opening, namely, an aperture in the bundle 
tissues just below the insertion of each leaf. 
In some plants this is only a small opening just 
below the nodes or places where the leaf sepa- 
rates from the stem. To this class belong, for 
the most part, plants whose stems are small 
creeping rhizomes with alternate two-ranked 
leaves. Fic. 42. 
The greater number of ferns with upright Course of vascular 
bundles in stem of 
Blechnum boreale. 
differ from the above type in having larger ¢ eashags heres 
: ‘ vascular bundle 
openings separated from each other by relatively = running down- 
smaller or thinner bands of vascular tissue, so fre toward the 
eaves, slightly 
that the hollow cylinder formed by the bundle — magnified. — 
(Aced, to Unger.) 
stems, leaves in many rows, and short internodes, 
tissue appears to consist of a network whose 
meshes are the so-called leaf openings. As this type is of 
frequent occurrence, a description of a single stem may be 
given here in order to make the manner of construction clear. 
The young plant of Aspidium Felix-mas begins its growth 
with leaves arranged with an angular divergence of one-third, 
and with their separate bundles united sympodially into an 
axillary stem bundle. Having reached the fifth or sixth leaf, 
