PHLOEM AND XYLEM. 15 



constitution is, however, as yet unknown. It is harder and more 

 elastic than cellulose, readily permeable by water, but not absor- 

 bent, not, that is, retaining the water. It is more soluble in 

 acids, such as chromic acid, than is cellulose, and is recognized by 

 turning brown when treated with Schulze's solution, a mixture of 

 zinc-chloride, potassium-iodide, and iodine which turns unaltered 

 cellulose blue. 



The elements of the phloem, with which we are less concerned 

 than we are with the xylem, though often variously thickened, 

 are not lignified. They consist of bast-parenchyma, sieve-tubes, 

 companion-cells, and bast-fibres, besides the medullary rays which 

 traverse xylem and phloem alike. Bast parenchyma consists of 

 slightly elongated cells in vertical rows of four or six, of which 

 the terminal cells taper. This arises from each row having been 

 formed by several transverse divisions of a single procambium or 

 cambium cell. They generally contain protoplasm and sometimes 

 grains of starch or crystals. Sieve-tubes are the vessels of the bast, 

 long tubes with transverse partition-walls and retaining their 

 protoplasm but communicating through these transverse walls by 

 the sieve-plates from which they take their name. The sieve-plate 

 is a thin portion of the wall perforated by numerous pits close 

 together. The sieve-tubes are the chief channel by which proto- 

 plasmic matter manufactured in the leaves is conveyed through 

 the stem. Companion-cells occur only in angiosperms. In longi- 

 tudinal section they appear as narrower cells alongside the sieve- 

 tubes filled with granular protoplasm and with unperforated 

 transverse walls adjoining those of the sieve-tubes. In a trans- 

 verse section they appear like small corners cut off the larger 

 sieve-tubes, and they have their name from the fact that each of 

 them originates in this way, a longitudinal wall dividing the 

 original cell into two unequal parts, of which the larger contributes 

 to a sieve-tube, the smaller remains a cell. Bast-parenchyma, 

 sieve-tubes, and companion-cells are known collectively as soft bast 

 in contradistinction to bast-fibres or hard bast. Bast-fibres are 

 extremely elongated structures, tapering at each end, containing 

 only water or air, and with their walls so thickened as sometimes 



