20 



rind or protective covering of thick-walled cells more or less 

 impervious to water, and bearing the matter which gives the 

 twig its color. Within this is a broad, loose belt of green cells 

 and large intercellular spaces which reach to the tough fibrous 

 band of bast. Succeeding the bast, passing inward, is an- 

 other loose belt of cellular tissue narrower than the one out- 

 side the bast into which the medullary rays project as they 



throusjh the cambium laver. Next, within this cam- 

 bium, is the wood with its medullary rays which reach to the 

 central cylinder of pith. The outer portion of the pith, the 

 meduUary rays, and the pith wood-cells at right angles to the 

 latter, are the three portions of the internodial part of a twig 

 that are starch-bearing. The above description considers only 

 sections that might be made through the twig at any point 

 between the buds. At the nodes or points where lateral buds 

 are developed there are certain modifications of the structure 

 which have been considered. 



J 



— At the point of union or junction of two 



years' growth of a twig, as of that found during 1889 ^^"^^ ^^"^^^ 

 of 1890, there are but few structural features not included in. 

 the description of the twig already given. The point is easily 

 determined from the outside, for the scars of the several scales 

 of the terminal bud of the previous year remain to mark the 

 place, which is somewhat larger in cross-section than the twig, 

 an inch or so above and below it. It also is a starting-point 

 from which the buds become more distant, proceeding either 

 up or down the twig. If these points are not sufficient, the 

 brighter, fresher red of the more recent growth will be quite 

 sure to distinguish the place of union. With a cross-section 

 of the twig under the microscope, the age in years is quickly 



^1 



determined. The first wood formed on a yearling twig in 

 spring is more porous than that last produced the previous 

 autumn. It is on this account that the wood of an old branch 

 is arranged in evident rings, and if there has been no interrup- 

 tion in the growth of the plant during any summer there will 

 be a ring for each year, the thickness and porosity of which 

 will vary with the season's favorableness for growth. Micro- 

 scopic inspection of the juncture does not reveal any very 

 marked modification of structure. The cvlinder of pith is 



