THE ORGANIZATION OF THE PLANT 



67 



FIG. 45. Lilac 

 buds 



The sectioned spec- 

 imen shows the em- 

 bryo leaves and 

 stem 



between the bundles are called medullary rays. These serve 

 to transport foods across the stem. The activities of the 

 cambium annually add new layers to both 

 the wood and bast. In consequence dicoty- 

 ledon stems yearly increase in diameter. In 

 the new wood new ducts are also formed, 

 and these circles of ducts serve to distinguish 

 the wood of one season from that of another. 

 Monocotyledons, on the 

 other hand, lack cambium 

 and commonly do not in- 

 crease in diameter after 

 the stem once starts upward. Externally 

 the two groups also present several notice- 

 able differences. The monocotyledons have 

 more conspicuous joints, seldom branch, 

 and, since they lack a cambium, have no 

 bark. Among the well-known monocoty- 

 ledons are sugar cane, rice, wheat, and all 

 the other grains and grasses, as well as such 

 plants as the lily, iris, and tulip. Our fruit 

 and forest trees and most of our garden 

 plants are dicotyledons. 



Buds. All ordinary stems increase in 

 length at the tip. At this point the rudi- 

 mentary stem is crowded with undeveloped 

 leaves, forming what is known as a bud. In 

 woody plants, in addition to this terminal 

 bud, other growing points may develop 

 along the sides of the stem, late in the 

 growing season, from which new twigs or 

 flowers arise the following year. These 

 are called lateral buds. They always occur at the joints of the 

 stem and just above a leaf. Extra buds, called accessory buds, 



FIG. 46. Naked buds 

 of viburnum 



