1 62 PLANT LIFE. 



water travels freely. The walls of these vessels are 

 generally pitted. Both the tracheids and vessels there- 

 fore are intended to conduct the water. Very often 

 the mass of xylem is strengthened by dead strong- 

 walled woody cells or fibres ; often also the xylem 

 contains living cells, especially when the medullary rays 

 are few in number. In the last case, these living 

 cells preserve a connection with the rays by means 

 of which they are supplied with food material. 



In the wood, or xylern^ the medullaiy ray is 

 formed out of a cambium cell, or group of cambium 

 cells. Each cell is divided by horizontal walls into a 

 tier of cells, of which the uppermost and lowest are 

 triangular, or of the shape of a Gothic arch ; these are 

 generally without protoplasm resembling tracheids in 

 thickening, and take in water which passes either into 

 the central row, or upwards into other tracheids. The 

 cells in the central row of the ray are full of protoplasm, 

 and often contain food-stores. 



The phloem part of the medullary rays only differs 

 in the lowest and uppermost cells retaining their proto- 

 plasm, which is well nourished from the phloem, so that 

 the whole medullary ray carries out food material to 

 the cortex, and cork cambium. 



Those cambium cells, which form the phloem, are 

 changed into sieve ttibes^ phloem cells, or fibres. The 

 sieve tubes are formed of curving, vertical rows of 

 these cells. The transverse divisions are not destroyed ; 

 but are merely pierced by numerous openings, so 

 that they look like a sieve. The phloem cells 

 are formed either by the division of cambium cells, 

 or they retain the original shape of the latter. 

 The fibres are pointed, very much elongated cells, 

 with extremely thick, woody walls, and are without 



