150 



BOTANY 



PART I 



unaltered. The differences in the appearance of the bast of dicotyledonous trees are 

 due to the greater or less diameter of the sieve-tubes, the presence or absence of 

 bast fibres, and the arrangement of the various elements. 



The Medullary Rays of the Gymnosperms (Fig. 149 ms) and 

 woody Dicotyledons (Fig. 155 jvn, sm) form radial bands, composed 

 wholly or in part of parenchymatous elements. Their function is to 

 supply the cambium and wood with the products formed in the leaves 



and conveyed away by the bast ; they 

 also conduct water outwards from the 

 xylem. The medullary rays in this way 

 link together by radial bands of living 

 cells the protoplasm-containing elements 

 of the bast and wood, thus uniting all the 

 separate living tissues of the stem. The 

 medullary rays are in turn accompanied 

 or, if many-layered, traversed by inter- 

 cellular spaces filled with air. These, 

 beginning in the periphery of the stem, 

 penetrate the cambium and communicate 

 with all the intercellular spaces through- 

 out the living elements of the wood and 

 bast. All the living elements are kept 

 in communication with the atmosphere 

 by means of the intercellular spaces 

 of the medullary rays, and the necessary 

 interchange of gases is thus rendered 

 possible. 



Tlie 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 iu the Pine, single 

 rows of cells, without living contents and 

 situated usually at the margin of the medul- 

 lary bands, become tracheidal in structure 

 (Figs. 152, 153 tm), and communicate with one 

 another and with the tracheides liy means 

 of bordered i)its. Their purpose is to facilitate the transfer of water radially 

 between the tracheides. In other Conifers, where such tracheidal elements are 

 not found in the medullary rays, bordered pits are developed in the tangential walls 

 of the tracheides of tlie autumn wood, and by means of them the transfer of water 

 in a radial direction is effected. The living cells of the medullary rays of the wood 

 bear the same relation to the water-carriers as do the cells of the wood parenchyma, 

 and like them are connected with the water-conducting elements by means of 

 bordered pits on one side of the cell wall. They take up water from them and 



Fig. 158. — Tangential spctiou of the wood 

 of Tilia idinifolta. m, Pitted vessel ; 

 t, spiral tracheides ; p, wood paren- 

 chyma ; I, wood fibres; r, medullary 

 rays, (x 160.) 



give it out again, as it may 



be needed to other living cells 



on the other hand. 



