78 BIOLOGY FOR BEGINNERS 



FUNCTIONS OF STEM TISSUES 



The tissues in order from without are the epidermis, cortex, 

 bast fibers (hard bast), bast tubes (soft bast), cambium, wood, 

 ducts, pith, and pith rays from center to cortex. Each of these 

 layers has its definite functions, several of which have been stated. 



Epidermis. The outer layer, or epidermis, is largely protective 

 and hi several ways. Its thickness guards against injury from wind, 

 weather, and attacks of insects. It does not allow loss of water, 

 except at the lenticels, thus preventing undue drying of the deli- 

 cate tissues beneath. It also keeps out the spores of parasitic fungi 

 that might otherwise find entrance and destroy the plant. 



Cortex. Under the epidermis is the cortex, whose function is 

 to help prepare starch food for the plant, much as do the 

 leaves. 



Bast Fibers. The bast fibers give toughness to the bark, some- 

 times helping support the stem. Man has taken advantage of 

 the fiber strength of hemp and flax (look up) to make fabrics. 



Bast Tubes. The soft bast conveys food prepared by leaves 

 downward to various places where it is used or stored. 



Cambium. The growth function of the cambium cannot be too 

 often mentioned, as from it, by a complicated process of cell divi- 

 sion, bark tissues on the outside and wood and ducts within are 

 formed. 



Ducts. The ducts transfer liquids up and air down in the stem, 

 and add their strength to the woody portions, whose fibers are the 

 chief support of the stems of all larger plants. Together they make 

 up the bulk of the stem tissue. 



Wood Fibers. Both the wood fibers and ducts are arranged in 

 very definite circles, called annual rings because usually each ring 

 marks a year's growth. These rings are caused by the cambium 

 which produces larger ducts and more of them in the spring when 

 the sap is flowing than later, when more wood fiber is produced. 

 In the winter, the growth practically stops, only to begin the fol- 

 lowing spring with a layer of large ducts again, thus marking, by 

 these successive rings of tissue, the seasons' changes. 



