140 



BOTAXY 



PART I 



traversed by intercellular spaces filled with air. These, 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 

 intercellular 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 woo. 1 

 parenchyma. In the medullary rays of certain 

 Gymnosperms, particularly in the Pine, single 

 rows of cells, without living contents and situ- 

 ated usually at the margin of the medullary 

 bands, become tracheidal in structure (Figs. 143, 

 144 (m), and united with one another and with 

 the tracheides by means of bordered pits. 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 the 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 does 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 give it out again, as it may be 

 FIG. 149. Tangential section of the wood needed, toother living cells ; on the other hand, 

 of Till,, ..tmifoli,,. m. Pitted vessel ; in the wring, at the beginning of the season 

 t, spiral traoheides ; j>, wood paren- . 



,-hyma; I, wood llJ.n-s ; r, m.Nlullary of gH"**, the J r press m * * he water-cour.ses 

 rays, (x 160.) the products of assimilation, in particular 



glucose and small quantities of albuminates, 



in order that these substances may be transferred in the quickest way to the points 

 of consumption. Accordingly, during the winter and in the beginning of spring, 

 sugar and albumen may be detected in the tracheal elements ( }26 ), and may thm 

 be obtained from the watery sap of "bleeding " trees, or from artificial borings or 

 incisions, particularly in such trees as the Maple, Birch, and Hornbeam. In the 

 wood of Dicotyledons it is usually only special rows of the medullary ray cells 

 which stand in such close relation with the tracheal tissues. In these special rows, 

 generally on the margins of the medullary rays, the cells- are elongated vertically, 

 and on that account have been distinguished as VERTICAL MEMUI.LAUY KAY CELLS. 

 Theiother cells, or those of the middle layers of the medullary bauds, on the other 

 hand, are called HORIZONTAL MEDULLARY KAY CELLS ; they are narrower and 

 more elongated radially ( I27 ). These have no especial connection with the tracheal 



