304 



ECOLOGY. 



form, size, color, and armature nf the buds, as well as great variations in 

 the character of the bud scales. There are striking dififerences between the 

 buds of different genera, and with careful study differences can also be seen 

 in the members of a genus. 



571. Growth in thickness of woody stems. — In the growth of woody per- 

 ennial shoots, the shoot increases in length each j-car at the end. The 



shoot also increases in diameter each 

 \'ear, though portions of the shoot one 

 year or more old do not increase in 

 length. We can find where this 

 growth in diameter of the stem takes 

 place by making a thin cross section 

 of a young shoot or branch of one of 

 the woody plants. If we take the 

 white ash, for example, in a cross 

 section of a one-year-old shoot we 

 observe the following zones : A cen- 

 tral one of whitish tissue the cells of 

 which have thin walls. This makes 

 a cylindrical column of tissue through 

 the shoot which we call the pith or 

 medulla. Just outside of this pith 

 is a ring of firmer tissue. The inner 

 portion of this ring shows many 

 woody vessels or ducts, and the outer 

 portion smaller ducts, and a great 

 many thick-walled woody cells or 

 fibres. This then is a woody zone, 

 or the zone of xylem. 



572. The outer ring is made up of 

 the bark, as we call it. In this part 

 are the bast cells. Between the bark 

 and the woody zone is a ring of small 

 cells with thin and delicate walls, and 

 richer in protoplasm, 

 these cells 



are apt to show a deeper color than either the wood zone or the bast zone. 

 This is, as we will recollect from our study of the bundle in stems, the cam- 

 bium zone, or the growing part of the older portions of the stem. 



573. We may wish to know why these portions of the bundle here form a 

 continuous or apparently a continuous ring in the stem of a \\'oody plant. In 

 the study of the sunflower stem, and also of impatiens, attention was called 

 to the increase in the number of the bundles as the stem increased in age. 



Fig. 400, 

 Three-year-old twig of the American ash, with ti. p rplls 

 sections of each year's growth showing annual 

 rings. If the section is stained. 



