INTRODUCTION TO PLANT HISTOLOGY 467 



Xylem Parenchyma 



Apart from tracheids and vessels the woody tissue contains numbers 

 of short Hgnified cells usually called xylem parenchyma. This term, 

 xylem, covers the aggregate of woody vascular tissues, and we shall meet 

 it constantly henceforth. The xylem parenchyma have very numerous 

 simple pits. They may be either dead or living, and in the latter case may 

 contain stored starch. They occur in three different ways in different 

 woods. They may be generally distributed among the other elements, they 

 may be grouped around the vessels or they may be formed along with the last 

 xylem cells produced at the end of the growing season. The type of 

 distribution is helpful in identifying species of timbers. 



Sieve Tubes 



The non-lignified prosenchyma constitutes another type of vascular 

 tissue. The chief example is the sieve tube. Sieve tubes are longi- 

 tudinal files of elongated, thin-walled cells, the transverse walls between 

 them being pierced by numerous pores, which gives them the name of 

 sieve plates. 



In its early state a sieve tube element is a typical living cell, with proto- 

 plasm showing cyclosis, but during development the movement ceases, 

 the nucleus disappears, the cytoplasm becomes coarse and permeable, the 

 distinction between vacuole and utricle becomes blurred, and probably the 

 mature cell eventually dies. The vacuole contains a strand of slime, which 

 often appears to be accumulated at the sieve plates into the so-called slime 

 plugs. These plugs are probably only artificial appearances and are not 

 seen in uninjured cells. 



The pores of the sieve plate are really open perforations through which 

 the protoplasm of neighbouring elements makes direct contact. They are 

 therefore extremely large plasmodesma. Whether the slime also passes 

 through the pores is still uncertain, but there can be no doubt that the sap 

 can pass freely through the highly permeable protoplasm. The mature 

 sieve plate has usually linings to the pores formed of a polysaccharide sub- 

 stance called callose, which gradually increases in amount until it forms a 

 thick pad over the plate. The protoplasmic connections persist through the 

 pad for some time, but are eventually broken. Such pads may form during 

 the first winter and be dissolved in spring, but in such cases the second 

 winter brings the formation of a permanent pad and the end of the tube's 

 functional life. Sieve fields, similar in structure to the sieve plates, may 

 often occur on the side walls of the tube where it is contiguous with 

 another sieve tube (Fig. 455). Sieve tubes do not serve simply for 

 the passage of water as such but are conductors of carbohydrates and 

 nitrogenous food materials. 



Associated with each sieve tube is a companion cell, which is derived 

 by division from the same mother cell as the sieve tube element alongside 



