Tinnll^^irdfri'Ta^ Micvoscoinc Struduve of Plants. 159 



fibro-vascular bundles. Sometimes they are very easily separated 

 fifom the tissues in which they are imbedded, as in Plantago, while 

 in others they can only be separated with great difficulty. In many 

 fruits and stems the delicate cellular tissue disappears, or dries up, 

 and a number of string-like fibro-vascular bundles are left. In 

 Myriophyllum the bundles are softer than the surrounding tissues, 

 and apparently cannot be isolated. The bundles are often sepa- 

 rated ; but in some stems, as in those of Conifers and Dicotyledons, 

 the bundles are compressed together in such a manner that the 

 separate bundles of which the entire mass is composed are no longer 

 distinguishable. Each of the fibro-vascular bundles consists of 

 various different forms of tissue, forming a system running through 

 the entire plant. At first the bundles consist of masses of similar 

 cells, with no intercellular spaces. This tissue has been called the 

 Procambium. As the procambium grows older, then the cells 

 become more or less modified, and converted into the various 

 permanent forms of cells, vessels, bast, &c., that we find in the 

 perfect bundle. It frequently happens that the whole of the pro- 

 cambium is converted gradually into permanent tissue; but at 

 times an inner layer of the procambium remains active, forming 

 the cambium layer. In some plants we have the fibro-vascular 

 bundles containing cambium, while in others there is no cambium. 

 The bundles containing cambium are open or indefinite ; those 

 containing no cambium are closed or definite bundles. Closed 

 bundles are to be met with in the Cryptogams, Monocotyledons, and 

 many Dicotyledons, and are incapable of growth beyond a certain 

 size. Open bundles, on the other hand, are to be met with in the 

 roots and stems of Dicotyledons and Conifers, and are capable of 

 growth for an indefinite period. In the leaves of Conifers and 

 Dicotyledons we generally find closed bundles ; but if they are 

 open, then their activity soon ceases, and growth stops. Naegeli 

 has divided the tissues of the vascular bundles into two groups. 

 These are separated by the cambium, and are named Phloem and 

 Xylem ; the Xylem part being the wood proper, while the Phloem 

 is in general considered as part of the bark. It is, however, deve- 

 loped as part of the vascular bundle, distinct from the hmitary 

 and primitive tissues. The ceUs in the Phloem part of the fibro- 

 vascular bundle are thin-waUed sap-bearing cells and the thickened 

 bast-cells, while the Xylem consists of cells which have become 

 thickened and woody. Sometimes, as in the Kaddish, Potato, &c., 

 the cells of the Xylem do not become woody, but remain thin- walled 

 and parenchymatous. The medullary rays belong to both the 

 Phloem and Xylem part of the bundle. The structure of the fibro- 

 vascular bundles is not difficult to examine, and many forms of cells 

 and vessels enter into their construction, rendering them interesting 

 objects of study. 



