MORPHOLOGY 



125 



Fig. 13S. — Transverse section of a vessel from 

 the heart-wood of Robinia Pseudocode, closed 

 by tyloses ; at a, a is shown the connection 

 between the tyloses and the cells from which 

 they have been formed, (x 300.) 



L. (Campeachy wood, logwood), with a blue heart-wood from which hematoxylin 



is extracted ; Pterocarpus santalinus, L. fil. (red sandal- wood), from the heart-wood 



of which saxtalin is obtained ; Caesal- 



pinia brasiliensis, L. , and 0. echinata, 



Lam. (Brazil wood, Pernanibuco wood), 



with a red heart-wood which supplies 



erasilin ; and the Alsage Orange, 



Madura aurantiaca, Nutt. (yellow Brazil 



wood), which has a yellow heart-wood 



from which mokin is derived. 



Tyloses (Fig. 138) are also instru- 

 mental in closing the water-courses of 

 the heart-wood. These are intrusive 

 growths from living cells, which pene- 

 trate the cavities of the adjoining 

 tracheal elements during the transition 

 of the sap-wood into heart-wood. In 

 the formation of tyloses the closing 

 membrane of the pits of pitted vessels 

 forms bulging ingrowths into the vessel 

 cavities. Such bulging ingrowths in- 

 crease in size until several meet, and so 

 more or less completely close the cavities 

 of the vessels into which they have in- 

 truded. 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. 

 Mays,, resin does not penetrate the walls of wood cells under normal conditions ; 

 the wood of Conifers only becomes resinous through the impregnation of the cell 

 walls with resin, alter they have become dried up through wounds or other causes. 

 The resin-ducts of Conifers may also be closed by the formation of tyloses. 



The elements of secondary growth in Gymnosperms and Dicoty- 

 ledons differ. The vascular strands of Gymnosperms are composed 

 almost exclusively of tracheids (Fig. 139). These are provided with 

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

 walls. With the exception of the genus Ephedra, 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 tracheids, the number of which is occasionally 

 doubled by the radial division of a cambium cell (Fig. 139, a). The 

 tracheids of the early wood (/) are distinguishable from the late 

 tracheids (s) by their larger lumina. 



In the Pine, the early as well as the late tracheids have bordered pits in their 

 radial walls only ; while in other Conifers they are present also in the tangential 

 walls of the later-formed tracheids. The bordered pits in the early tracheids are 

 not only more numerous, but also larger than those in the later tracheids (Fig. 

 141 t). The tracheids are often over a metre long, much longer than the cambium 

 cells from which they are developed. They attain this length by a subsequent 

 orowth, during which their growing ends become pushed in between one another. 



