134 



BOTANY 



and conveyed away by the bast. The medullary rays in this 

 way link together by radial bands of living cells the protoplasm-con- 

 taining elements of the' bast and wood, thus uniting all the separate 

 living tissues of the stem. The medullary rays are in turn accom- 

 panied or, if many -layered, traversed by intercellular air-cavities, 

 which, beginning in the periphery of the stem, penetrate the cambium 

 and communicate with all the intercellular spaces throughout the 

 living elements of the wood and bast. All the living elements are 

 kept in communication with the atmosphere by means of the inter- 



Fig. 149. — Portion of a transverse section of the bast of Tilia parvifolia. v t Sieve-tubes ; v*, sieve- 

 plate ; c, companion cells ; k, cells of bast parenchyma containing crystals ; p, bast paren- 

 chyma ; I, bast fibres ; r, medullary ray. ( x 540.) 



cellular spaces of the medullary rays, and the necessary interchange 

 of gases is thus rendered possible. 



The substances contained in the parts of the medullary rays 

 within the wood, chiefly consisting of starch, tannins, resin, and crystals, 

 are essentially the same as those in the wood parenchyma. In the 

 medullary rays of certain Gymnosperms, particularly in the Pine, 

 single rows of cells, without living contents and situated usually at 

 the margin of the medullary bands, become tracheidal in structure 

 (Fig. 141, tm), and united with one another and with the tracheids by 

 means of bordered pits. Their purpose is to facilitate the transfer of 

 water radially between the tracheids. In other Conifers, where such 

 tracheidal elements are not found in the medullary rays, bordered 

 pits are developed in the tangential walls of the tracheids of the late 



