vin THE HISTOLOGY OF THE TISSUES 99 



some cases have sharp-pointed ends. Prosenchyma cells are 

 also found mixed up with the vessels (p. 92). 



In addition to the vessels and the fibrous cells a number of 

 parenchyma cells occur mixed with the vessels ; these paren- 

 chyma cells never fuse together. 



Phloem. The phloem consists of two portions which are 

 known as the soft- and hard-bast. The soft bast is found 

 close to the cambium and consists of sieve tubes, companion 

 cells, and parenchyma cells. The sieve tubes (p. 93) are long 

 vessels which have their transverse walls perforated. Com- 

 panion cells which can always be recognised in a longitudinal 

 section of the bast because they are long narrow cells rilled 

 with protoplasm, and each possessing a large nucleus are 

 found with the sieve-tubes. In transverse sections companion 

 cells appear as if they were originally cut off from the same cells 

 as the sieve-tubes. Mixed up with the sieve-tubes and com- 

 panion cells a few parenchyma cells may be found ; these are 

 known as phloem parenchyma. 



The hard bast is composed principally of bast fibres, which 

 are long narrow spindle-shaped fibres, much like the fibres of 

 wood. Parenchyma cells are to be found mixed with the bast 

 fibres. 



Cambium. The cambium is found between the xylem and 

 phloem. It consists of cells which do not as yet show the 

 characters of either xylem or phloem. Those cambium cells 

 nearest to the phloem pass gradually into it, while those nearest 

 the xylem eventually become the xylem. The cells near the 

 middle of the cambium are thin walled, and contain protoplasm. 

 They are in a state of constant division, and thus form new 

 cells. The new cells on one side pass into and form new 

 xylem. Similarly, on the other side new phloem is produced. 

 A tissue composed of cells which can divide in this way is 

 called meristematic, because it is capable of dividing up 

 and producing new cells. 



Open and Closed Vascular Bundles. Those vascular 

 bundles which possess a cambium are said to be open because 

 they can produce new tissue. If the bundles consist of xylem 

 and phloem only, without a cambium, they are termed closed 

 bundles, because growth in thickness of the bundle cannot go 

 on. When the xylem and phloem are in contact on one side 

 only, they are said to be collateral. 



H 2 



