134 BOTANY I-AHT i 



1 1 cart -wood. These are intrusive growths from living cells, which i>cnctr;itc tin- 

 cavities of the adjoining tracheal elements during the transition of the sap-wood 

 into heart-wood. In the formation of tyloses the closing membranes of the pits of 

 pitted vessels form bulging ingrowths into the cavities of the vessel. Such 

 ingrowths increase in size until several meet, and so more or less completely close 

 the cavities of the vessels into which they have intruded. The closing membrane 

 of the bordered pits in the heart- wood is pushed to one side, so that the torus presses 

 against the opening of the pit and completely closes it. According to H. 

 MA.YH ( 12B ), resin does not penetrate the walls of wood cells under normal con- 

 ditions ; the wood of Conifers only becomes resinous through the impregnation 

 of the cell walls with resin, after they have become dried up through wounds or 

 other causes. The resin-ducts of Conifers may also be closed by the formation <>t 

 tyloses. 



The elements of secondary growth differ in Gymnosperms and 

 Dicotyledons. The vascular strands of Gymnosperms are composed 

 almost exclusively of tracheides (Fig. 141 A}. These are provided 

 with bordered pits which are situated, for the most part, in their 

 radial walls. The tracheides of the spring-wood (/) have larger 

 cavities than those formed later (s). Parenchyma is also present in 

 the wood, though in relatively small amount ; in some Abietineae 

 resin-passages occur in it (Fig. 141 h). 



Except in the Gnetaceae, true vessels are not found in the secondary growth, 

 nor in the primary vascular portions,, of the bundles of Gymnosperms. The wood 

 produced by the cambium consists of radial rows of tracheides, the number of 

 which is occasionally doubled by the radial division of a cambium cell (Fig. 

 141 A, a). The tracheides are often over a millimetre long, much longer than 

 the cambium cells from which they are developed. They attain this length by 

 a subsequent growth, during which their growing ends become pushed in between 

 one another. In addition to the tracheides, small amounts of wood parenchyma 

 are also produced in Gymnosperms by a transverse division of the cambium cells. 

 It is in the parenchymatous cell rows of the wood of Pines, Spruce-Firs, and 

 Larches that the schixogenous resin-ducts are produced (Fig. 141 A, h). In other 

 Conifers the wood parenchyma consists of simple rows of cells, which afterwards 

 become filled with resin. 



Besides tracheides (t) and wood parenchyma (hp), other element > 

 take part in the composition of the secondary wood of a Dicotyledon ; 

 these are the vessels (tracheae, g), and the wood fibres (h) (Fig. 

 145 A, B}. The cells of the wood parenchyma are short and have 

 abundant contents, the woods fibres are thick- walled, long cells with 

 pointed ends. The elements with wider lumens, especially the 

 vessels, are abundant in the spring-wood, in which water conduction 

 is important. The autumn -wood, on the other hand, consists of 

 narrow elements, among which the wood fibres, which contribute to 

 the rigidity of the plant, are numerous. On account of these 

 differences between spring and autumn wood the annual rings are 

 well marked (Fig. 148). 



