98 GENERAL BOTANY 



xylem, increase greatly in diameter and undergo marked 

 changes in the character of their cell walls. This latter change 

 consists in the formation of the so-called sieve plates by perma- 

 nent perforations of the transverse walls between a vertical row 

 of cells which unite to form the sieve tube. 



The sieve tubes in the permanent condition are therefore like 

 water ducts in being composed of a vertical chain of greatly 

 enlarged cells adapted to the rapid conduction of materials for 

 the use of the plant. In the case of the sieve tubes the materials 

 conducted are soluble nitrogenous foods, with probably some 

 sugars and other substances. The perforations in the sieve 

 plates allow this material to move with less obstruction than 

 would otherwise be possible. The sieve tubes, unlike the water 

 ducts, retain their cytoplasmic contents during their active life 

 but lose their nuclei. They are connected with the companion 

 cells by perforated areas in the side walls, resembling somewhat 

 the sieve plates. 



Wood rays. The wood-ray cells arise from the cambium, like 

 the other tissues of the xylem and phloem, but undergo less 

 change in form and structure than the sieve tubes and the 

 ducts. They are really horizontal cells of the phloem and xylem, 

 corresponding in the main, in structure and function, to the living 

 cells already described. 



The above account illustrates the similarity between the grow- 

 ing layers, or meristems, in all parts of the plant body and the 

 developmental stages of cells which become transformed from the 

 meristem condition into the permanent tissues of the plant. 



THE GENERAL FUNCTIONS OF THE STEM TISSUES 



Storage. If transverse sections of living twigs or stems are 

 cut in late autumn or winter and stained with iodine solution, 

 the abundant storage of starch can easily be demonstrated. 

 This starch storage occurs mainly in the thin-walled cells -of 

 the cortex, phloem, xylem, and pith. In the xylem, or wood, 

 the starch appears abundantly in the wood-ray cells and in the 

 wood parenchyma. These cells often form continuous bands of 



