324 



GENERAL SCIENCE 



FIG. 285. Grains of Wheat 

 Growing Toward the Light. 



Stems vary in structure. In corn the stem or stalk is 

 pithy with a hard rind on the outside and numerous 



woody fibers running length- 

 wise through it. These fibers 

 and the rind are traversed by 

 minute holes, which serve as a 

 passageway for the sap. If a 

 longitudinal section of the stalk 

 is made, some of those little 

 fibers will be found to enter the 

 leaves at the joints. In the 

 leaves they appear as veins and 

 may be traced readily. The 

 tough woody cells of the rind 

 furnish the main support of the 

 corn stalk as it matures. 



The stem of the ordinary tree contains a very small 

 pith center. Surrounding the pith is the woody fiber, 

 which is composed of many little tubes and their woody 

 walls. If the stem is several years old, we can see dis- 

 tinct annual rings in this woody part. The space between 

 two consecutive rings indicates the growth during one 

 summer (Figures 286, 287). Radiating from the center 

 of the stem and extending to the bark are found a number 

 of tiny lines. These lines are called the medullary rays 

 and serve as storehouses for food and permit the flow of 

 sap across the rings of the tree. These rays produce the 

 beautiful effects that we see in quarter-sawed lumber. 



The soil water that is taken up by the root hairs, under 

 the influence of osmotic pressure and capillarity (root 

 pressure), is carried by the minute tubes in the stems to 

 all parts of the plant or tree. In a large tree most of the 

 sap is carried by the tubes in the sapwood. This is the 



