INTEKNAL ANATOMY OF ORGANS. STEMS. 



531 



In the Coniferae there is a total absence of ducts, the wood being formed 

 exclusively of prosenchyma with the peculiar bordered pits (fig. 537), or, 

 as in Taxus, with both pits and a spiral fibre (fig. 542). 



Annual growth of Rings. With the commencement of a second season 

 of growth, a Dicotyledonous stem begins .to acquire its especial peculiari- 

 ties. When the buds open to produce new shoots, cell-division recom- 

 mences in the cambium -region of the old bundles, and an additional layer 

 of wood is added gradually during the season to that formed the year be- 

 fore. Season after season this process is repeated, and thus the cross 

 sections of the stems present a series of concentric laminee of wood corre- 

 sponding to the number of seasons during which the stem has existed 

 (figs. 579 & 580). 



Fig. 579. 



Fig. 580. 



Fig. 579. Diagrams of cross sections of a one-year (A), two-year (B), and three-year old (C) 

 Dicotyledonous stem, or of the branches 1, 2, and 3 of fig. 580. The letters to A 

 indicate : b, suberous layer ; e, cellular envelope running into the black medul- 

 lary rays ; I, liber ; c. cambium- region ; n, wood ; p, medullary rays ; m, medul- 

 lary sheath of spiral vessels ; p, pith. The figures to C, 1, 2, 3, mark the wood and 

 liber belonging to the 1st, 2nd, and 3rd year. 



Fig. 580. Diagram of a vertical section of a Dicotyledonous stem 3 years old, with 3 branches 

 marked 1, 2, 3, indicating the age in years of the branch and the internode below 

 it. The figures below denote the ages of the layers of liber and wood ; p, pith ; 

 c, cambium ; m, medullary sheath ; w c, layers of wood ; I, layers of liber ; 

 6, cellular and corky layer of bark. 



The concentric lamellae of wood in Dicotyledons are really annual rings 

 in most trees of temperate climates. In the tropical trees it frequently 



