HOW PLANTS GROW 



34i 



We also observed that the general direction of growth 

 of the stem is upward, opposite to that of the first roots. 

 The stem takes a vertical position, that is, vertical to 

 the horizontal plane. The trees on a hillside do not stand 

 perpendicular to the slope, but perpendicular to the hori- 

 zontal plane of the earth. The branches from the main 

 stem grow outward as do root branches. The 

 roots grow outward to get food, while the 

 branches from the stems grow outward to get 

 light. 



If we examine the structure of a cornstalk 

 and a limb of a tree, we notice a marked dif- 

 ference. The cornstalk has a hard rind and 

 a large pith with hard, stiff fibers running 

 through it. There is no true bark like that 

 on a tree or like that on flax. The micro- 

 scope shows that these threads in the pith 

 and also in the rind of the cornstalk are full 

 of holes running lengthwise, through which 

 the sap flows. 



The cross-section of the limb of a tree contains a very 

 small pith in the center and a true bark on the outer 

 edge and hard wood between the pith and bark. We 

 can see distinct rings in the woody part; these are the 

 annual rings and the space between any two rings is the 

 wood that grew during one summer. There are also lines 

 or rays running out from the center to the bark, much 

 like the spokes of a wheel. These lines all extend to the 

 bark, but they do not all extend to the pith because a 

 few of the rays are started in each annual ring and are 

 continued after they are once started. These rays serve as 

 storehouses for food and for the flow of sap across the tree. 



If we examine with a magnifying glass the smoothly 



SPLIT STEM OF 

 A TREE 



p, pith; h, 

 hard wood; s, 

 sap wood; b, 

 bark. 



