IO BOTANY. 



19. Opening of the buds in the spring. — As the buds 

 "swell" in the spring of the year, when the growth of the 

 young leaves and of the shoot begins, the bud scales are thrown 

 backward and soon fall away as the leaves unfold, thus leaving 

 the "ring scar" which marks the start of the new year's 

 growth in length of the shoot. 



20. Variations in different shoots. — A study of a number of 

 different kinds of woody shoots would serve to show us a series 

 of very interesting variations in the color, surface markings, out- 

 line of the branch, arrangement of the leaves and consequently 

 different modes of branching, variations in the leaf scars, the form, 

 size, color, and armature of the buds, as well as great variations 

 in the character of the bud scales. There are striking differences 

 between the buds of different genera, and with careful study 

 differences can also be seen in the members of a genus. 



21. Growth in thickness of woody stems. — In the growth of 

 woody perennial shoots, the shoot increases in length each year 

 at the end. The shoot also increases in diameter each year, 

 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 central one of whitish 

 tissue the cells of which have thin walls. This makes a cylin- 

 drical 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. 



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 distinguished from the bark and the 

 woody inner portion by the finer texture of the cut surface. 



