STEXJCTURE OP THE STEM. 



53 



(6) The narrow lines running in very young stems pretty straight from 

 pith to hark, in older wood extending only a little of the way from centre 

 to bark, the medullary rays, shown in Fig. 40. i 



(c) The wedge-shaped masses of wood between these. 



(d) The holes which are so grouped as to mark the divisions between 

 successive rings. These holes indicate the cross-sections of vessels or 

 ducts (§ 82). Note the distribution of the vessels in the rings to which 

 they belong, compare this with Figs. 40, 41, and decide at what season 

 of the year the largest ducts are mainly produced. Cut off a grapevine 

 several years old and notice the great size of the vessels. Examine the 

 smoothly planed surface of a billet of red oak that has been split through 

 the middle of the tree 



(quartered oak), and note 

 the large shinmg plates 

 formed by the medullary 

 rays. 



Look at another stick 

 that has been planed away 

 from the outside until a, 

 good-sized flat surface is 

 shown, and see how the 

 medullary rays are here 

 represented only by their 

 edges. 



-Cross-Section of Oak "Wood as seen with 

 the Magnifying Glass. 

 J, Jf the annual rings.^ 



II. 



76. Details of Structure; Cross-Section. — Cut from shoots of the 

 apple tree, ranging in age from one to five years, a number of sections. 

 These should be as thin as they can be made without breaking up. It 

 will save time to make at one time a good many sections of any woody 

 part of the plant that is to be examined. ^ 



For examination with the lowest powers, cylinders J to -J- inch long 

 cut smoothly from the twig to be examined, and viewed as opaque objects, 

 will answer well. 



1 These and many other important things are admirably shown in the thin wood- 

 sections furnished for $4 per set of 24 by K. B. Hough, Lowville, N. Y. 



2 The shading in fine lines at J would be rendered more naturally by dots. 



s If time allows, the students should out their own sections : frequently this will 

 be impracticable. Sections not needed for the current lesson may be put in 50 per 

 cent alcohol or other preservative fluid in wide-mouthed bottles carefully labeled and 

 kept for future use. For a list of sections see Appendix 0. 



