STEMS AND BUDS 



55 



which grow for only one year, food is stored in the pith. The 

 wood or xylem tissue carries the water through the stem to 

 the leaves and forms the chief part of a woody stem but only 

 a small part of an annual plant, or even in a one-year-old 

 woody stem, as illustrated (Fig. 12) by the main tissues in 

 the tangential view of the first year's growth of the stem 

 much enlarged to show the cells in detail. The xylem cells 

 vary in shape and size both in longitudinal and cross section. 

 Some people believe that all the mineral salts go up through 

 the xylem, but others believe that a part of it goes through 

 the phloem. In trees large quantities of food may be stored 



in the xylem. 



The cambium is a layer of cells around the xylem. It 

 divides repeatedly during the life of the plant to add new 

 xylem cells on the inside and new phloem on the outside, 

 increasing in this way the diameter of the stem. The bark 

 consists of the several types of cells found outside the cam- 

 bium. The phloem which is next to the cambium carries 

 food material through its sieve cells from the leaves to other 

 parts of the plant. The cambium cells break when the bark 

 is removed from a woody stem during active growth. Out- 

 side of the phloem a stem usually has a cortex made up of 

 thin-walled cells in which food is stored. Older stems have 

 a band of varying thickness of corky cells on the outside to 

 prevent excessive loss of moisture. Younger stems have a 

 single layer of epidermal cells with cutin on the outside to 

 retard the loss of moisture. Since the outer layer of bark 

 sloughs off in pieces from older trees the cork becomes the 

 outside layer. 



For a few weeks in the spring, from the time the buds 

 begin to swell, the stems of most perennial plants grow in 

 length after which the cells thicken and become permanent 

 tissues. The leafy stems of twig C (Fig. 13) grew in the 

 spring and have ceased to grow in length. Other stems, such 

 as the tomato, geranium, petunia, and most of the other her- 



