140 the ligneous btstem. 



The outside-growers (Exogens), 

 The inside-growers (Endogens), 

 The point-growers (Acrogens), 

 The mass-growers (Thallogens). 



691. The exogenous structuek. A cross section of the stem or 

 brancli of any dicotyledonous plant (mustard, maple), exhibits zones of 

 different structures, which are distinguished as pith, medullary sheath, 

 wood, and bark. 



692. The pith occupies the central part of the stem. It consists of 

 parenchyma, is chiefly abundant in herbaceous plants and all young 

 stems. When new, it is filled with fluids for the nourishment of the 

 buds until they can make food for themselves. As the plant advances 

 in age, the pith loses its vitality, is fllled with air only, is often torn into 

 irregular cavities, or disappears. 



, 693. The medullary sheath immediately surrounds the pith. It 

 is a thin, delicate tissue consisting of spiral vessels. It communicates 

 with every bud, and sends off detachments of its vessels to the petioles 

 and veins of every leaf Its tubes secrete oxygen from carbonic acid or 

 water and convey it lo the Laves. 



694. The wood cousists of pleurenchyma and ducts (§ 666) ar- 

 ranged more or less distinctly in concentric zones or layers. The first 

 or inner layer, together with the medullary sheath and pith, is the pro- 

 duct of the first year. One new layer is formed each successive year, 

 during the life of the plant. 



695. Annual cikoles. The ducts are usually first formed and lie in the inner 

 part of the strata next the center, while the wood-fibers are produced toward tho 

 end of the season, and deposited in the outer part. The former are distinguished 

 by the large size of their open ends, while the fibers are minute and compact. This 

 circumstance renders the limits of each layer distinctly perceptible in a cross 

 section, and their number, if counted at the base, will correctly indicate the age 

 of the tree. 



696. Exceptions. There are doubtless some exceptions to this rule. In trop- 

 ical countries, where there is no distinction of seasons, there may bo several zones 

 deposited annually, or on the other hand, several or all the annual layers may bo 

 so blended by the uniform mixture of the ducts with the wood-tissue as to be itn- 

 distinguishable. The layers of the beet-root are certainly not annual. They seem 

 to correspond with the number of leaf cycles (§ 228). 



697. The alburnum and duramen — ^the sap-wood and heart-wood, 

 are well-known distinctions in the wood. The former, named from al- 

 hus, white, is usually of a light color and softer structure. It is the 

 living part of the wood through whose vessels mainly the sap ascends. 



698. How FORMED. The interior layers of the alburnum gradually 

 harden by the deposition of solid matter in their vessels, and the thicken- 

 ing of the cell-walls, until fluids can no longer pass through them. 



