54 PLANT LIFE. 



a bundle lie one behind the other in the same radial line, 

 the xylem innermost, the phloem nearest the circumference 

 of the stem. In most ferns, however, the arrangement of 

 the two elements of the bundle is concentric, the phloem 

 completely enclosing the xylem in a ring-Hke manner. The 

 fibro-vascular bundles of cryptogams and monocotyledons 

 never increase in size after their first formation, and are 

 said to be closed ; whereas the bundles of dicotyledons which 

 possess a layer of meristem or cambium between the xylem 

 and phloem, continue to increase in size from year to year, 

 due to certain of the new cambium cells becoming differ- 

 entiated respectively into xylem and phloem, such bundles 

 are said to be of en. 



The various modifications of cells forming fibro-vascular 

 bundles, also their relative number and arrangement, are 

 very varied in different groups of plants. The following is 

 the most general combination. Three kinds of cells are 

 met with in xylem and phloem respectively. 



In the Xylem. — (i) Vessels or cell-fusions having the walls 

 thickened in various ways, spiral, annular, reticulated, 

 scalariform, pitted, etc. True spiral and annular vessels 

 are only met with in the first year's wood (medullary sheath) 

 of open bundles ; the vessels of the wood of all succeeding 

 years being provided with pits on their longitudinal walls. 

 In some instances the transverse walls of certain vessels 

 are not entirely absorbed, but only pitted like the longi- 

 tudinal walls ; such are called tracheides, and closely resemble 

 wood-fibres. The diameter of vessels is usually much 

 greater than that of any other form of cell. Vessels contain 

 air or water. 



(2) Wood-fibres are very much elongated and of small 

 diameter, and their pointed ends overlap, the walls become 



