26 



HORTICULTURE FOR SCHOOLS 



FIG. 13. Cross-section of exogenous stem show- 

 ing annular rings of wood. 



bium forms a layer of bark inside the old bark. As a ring of 

 wood is added every growing season, the age of a section of 

 a stem can be told approximately by counting the rings. 

 Because the stem is increased in diameter by layers of wood 



on the outside, this type 

 of stem is said to be 

 exogenous (Fig. 13). Most 

 trees are exogens. It is 

 important to remember 

 that in exogenous stems, 

 the solutions pass upward 

 through the wood or 

 xylem inside the cam- 

 bium layer, while most 

 of the passage downward 

 is in the bark or phloem 

 just outside of the cam- 

 bium layer. 



Another type of stem is that in which the vascular bundles 

 are not arranged in circles but are scattered throughout the 

 stem (Fig. 14). Each vascular bundle is similar in structure to 



those already mentioned and the 

 solutions pass through them in the 

 same way, but rings of wood are 

 never formed. Because the growth 

 takes place on the inside,such plants 

 are called endogens, and include 

 palms, corn, and all true grasses. 



Stems store food materials for 

 future use, usually in the forms of 

 starches, which are insoluble. It 



FIG. 14. Cross-section of endoge- Ql^,,! J U A vommYiKprAr} that fnnrl 

 nous stem showing scattered ar- SnOUld I 

 rangement of vascular bundles. materials can be transported by the 



plant only in soluble forms, such as sugars, but that they are 

 changed into starches in order to be insoluble for storage. 



