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 

 the stem. There is usually less anas- 

 tomosing of such bundles with each other 

 than is the case in the open collateral type. 

 2 6 . This type is present in some of the Dicotyle- 

 doneae 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. 



