58 GROUPS OF TISSUES, OR TISSUE SYSTEMS 
ing cells, with usually a thick-walled bundle sheath about 
the whole. In some species of Lycopodium there are 
several plates of xylem alternating with phloem, with 
one bundle sheath around all. ‘Transitional forms are 
found between this type and the radial type of bundle on. 
the one hand and the collateral on the other. 
85. The collateral type of bundle is present in stems 
and leaves of Seed Plants, and of many of the Fern Allies. 
Three types may be distinguished, open collateral, 
closed collateral, and bicollateral. The first two differ 
in the presence or absence, respectively, of a layer of 
meristem cells (cambium) between the xylem and phloem, 
while the third type is characterized by the presence of 
a layer of phloem on the inner, as well as on the peripheral 
side of the xylem. ) 
86. The closed collateral type of bundles is especially 
characteristic of the class Monocotyle- 
doneae. It is usually associated, also, with 
a scattered arrangement of the bundles in 
J 
on 22.2”, 
OY tomosing of such bundles with each other 
ES than is the case in the open collateral type. 
ote .26-— This type is present in some of the Dicotyle- 
vascular bundle, d@oneae as well, but not so frequently as the 
open collateral type. As an example that 
may be easily obtained to study, the vascular bundle 
of Indian corn may be taken. In this the xylem portion 
shows, in cross-section, four (rarely three or five) large 
vessels, of which two (annular or spiral) are placed in 
the radial plane, and the other two (large pitted vessels) 
lie a little externally to and to the right and left of these 
two. Between these large pitted vessels, and bordering 
the outermost of the other two vessels, is a mass of smaller 
cells, sometimes fibrous, sometimes tracheary in nature. 
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