BUDS AND STEMS 



105 



rangement of leaves within 

 the bud is known as vernation. 

 An interesting piece of field 

 and laboratory work is the 

 comparison of vernation in a 

 number of different buds. 

 Those of the wild cherry, birch, 

 European walnut, snowball, 

 lilac, and tulip tree are of in- 

 terest for this purpose. 



Why buds are Covered. — 

 When we consider that most 

 of our earliest green leaves 

 come from opening buds in 

 the early spring, the impor- 

 tance of a protective cover- 

 ing is well seen. Nevertheless 

 buds are frozen time and again 

 during the cold weather, only 

 to thaw out again without in- 

 jury to the plant. Sudden 

 changes, however, do much 

 harm. Some buds do not open 

 during mild winter weather 

 when temperature conditions 

 are seemingly favorable; a 

 definite length of growth seems 

 in that case to be necessary. 

 During warm weather plants 



give rise to buds which are devoid of protective scale leaves. Such is also 

 noticed in tropical forms, which are not called upon to meet rigorous cli- 

 matic conditions. We have now 

 learned something about the outside 

 markings of a branch.^ 



Let us turn our attention to the in- 

 ternal structure of the horse-chestnut 

 stem to see how the parts within are 

 adapted to perform the work which 

 they do. 



Study of a Dicotyledonous Stem. — 

 In the cross section of horse-chestnut 

 twig, notice the innermost part, the 

 pith. See if it is as tough as the wood 

 of the stem. About what proportion 

 of the cross section is occupied by 

 the wood? Everything outside of the 

 wood is collectively known as the 

 Section across a young twig of box elder, bark. It is made up of three layers, 

 showing the four stem regions; e, epi- Pull off outer layer; notice Its color, 

 dermis, represented by the heavy bound- 

 ing hne; c. cortex; w, wood; p, pith. ^ See Hunter and Valentine, Manual. 

 (From Coulter, Plant Relations.) page 32. 



Opening bud of horse-chestnut; L., leaves; L.S., 

 leaf scar; S., scalelike leaves which cover bud. 



