STEMS AND THEIR FUNCTIONS 161 



sieve tube cell lacks a nucleus; this is the rule in angiosperms, in which 

 each sieve tube cell is accompanied by a companion cell of the same length 

 but smaller diameter, containing a nucleus and connected protoplasmi- 

 cally to the sieve tube cell through openings in the cell wall. A sieve tube 

 generally functions during only one season and ends by forming an 

 impervious callus plate over the end walls, closing the transport channel. 



With an increase in the thickness of the stem, the cambium produces 

 an ever increasing number of vascular rays composed of radially elongated 

 parenchyma cells. The rays are in the form of thin, vertical, radially 

 arranged bands or sheets of cells from a fraction of an inch to a few inches 

 in vertical extent and only a few cells thick. The same ray extends both 

 inward into the xylem and outward into the phloem, since it is produced 

 by the formation of new cells alternately on the two sides of the same 

 group of ray cambium cells. The vascular rays come into contact with 

 both phloem and xylem conducting tissues and provide a path for radial 

 transfer of water and solutes between these parts of the stem. In addition 

 to the ray parenchyma, phloem (and to a lesser extent xylem) contains 

 strands of parenchyma that serve for storage of food and metabolic 

 products. 



Water transport. The great volume of water needed to make good 

 the constant loss by transpiration from the leaves is carried almost 

 wholly by the tracheids and vessels of the xylem. The pith, cambium, 

 phloem, and cortex are scarcely involved. This can easily be demon- 

 strated by experiment. If the stem is "girdled" by removing the tissues 

 outside the cambium, including the phloem, the leaves do not wilt until 

 the plant begins to die from interference with food and mineral transport 

 in the phloem. But if the outer tissues are split on each side of the stem, 

 the phloem carefully separated from the xylem with as little injury as 

 possible, and the xylem then cut across leaving the phloem intact, wilting 

 follows at once. 



Any explanation of the rise of water (sap) in the xylem must account 

 for the large volume and rapid rate of movement, and for the height to 

 which it is lifted. A single maize plant loses over 50 gallons (more than 

 400 pounds) of water from its leaves during its 100-day life span, while a 

 large apple tree loses about 10 gallons every day during the growing 

 season. Such losses require rapid upward flow of water in the xylem. In 

 the trunks of ring-pored trees such as oak, ash, and black locust a rate of 

 upward flow of 13 inches per minute has been observed, with a maximum 

 of 30 inches per minute in some vessels. In diffuse-pored trees such as 

 apple, beech, maple, and basswood the rate in the trunk is about 3 to 

 4 inches per minute, while in conifers (which lack vessels) it is much 

 slower, only about 0.5 inch per minute. As regards the height to which 

 water may be raised, consider the American sequoia or the Australian 



