THE GROWTH OF ROOTS AND STEMS. 177 



(h) Another and simpler method is to use the long leaf-stalks of 

 Rhubarb or of " Arum Lily " (Richardia). Lay the stalk down, cut the 

 ends squarely, and measure the length carefully. Then remove a 

 strip of the outer tissue and measure : it will be shorter than the 

 whole stalk. Next strip off the whole of the outer tissue and measure 

 the pith, which will be longer than the whole stalk. 



() Hold a Rhubarb leaf-stalk by one end in a horizontal position ; 

 it is firm and rigid and hardly droops. Strip off all the outer tissue, 

 leaving only the central portion (pith) ; the stalk now droops when 

 held by one end (why ?). 



(j) Saw off a piece of Willow twig about 2 ins. diameter. Slit the 

 "bark" down one side and remove it in a single piece. On trying to 

 replace it on the wood, you will find that the ends will not meet now, 

 showing that the bark was in a state of tension. 



207. Growth in Thickness. In almost all Monocoty- 

 ledons the older parts of the stem are hardly any thicker than 

 the younger ones near the growing apex. This is because the 

 stem of a Monocotyledon usually does not undergo any con- 

 tinuous increase in thickness after growth in length has 

 ceased. In Dicotyledonous shrubs and trees, however, a com- 

 plete layer of growing tissue or cambium is present outside 

 the wood but inside the bast, and each year this forms a new 

 cylinder of wood outside the older one, also producing new 

 bast within that previously formed. What are (1) the 

 mechanical, (2) the nutritive, necessities which are met by 

 " secondary thickening " ? Eemember the conducting func- 

 tions of the wood- and bast-tubes, also that the fibres give 

 strength, while the ordinary (living) cells store food. 



The process of secondary growth in thickness is obviously correlated 

 with the continual increase of leaf-area on one hand and of root-area 

 on the other. Each bud which unfolds in spring produces a branch 

 which bears buds ; next year each of the latter may produce a branch, 

 and so on. Meanwhile the root-system is also branching, each branch 

 producing fresh crops of root-hairs. The increasing rates of transpira- 

 tion and root-absorption are met by the provision of additional water- 

 conducting tubes (vessels) ; while the increasing amount of organic 

 food produced by photosynthesis is conducted by the additional sieve- 

 tubes to buds, cambium, growing-points of root-branches, and to 

 medullary rays. All these tissues are growing in extent year by year, 

 their growth being intimately correlated. 



That the increasing weight of the foliage borne by the branches of 

 trees is practically counterbalanced by the increasing amount of wood 

 formed each year is shown by the fact that the vertical distance 

 between the drooping summer-position of a branch, when it is bent 



S.B. 12 



