MORPHOLOGY 135 



wood, and by means of them is effected the transfer of water in a radial 

 direction. 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. They take up water from them and give it out 

 again, as it may be needed, to other living cells ; on the other hand, in 

 the spring, at the beginning of the season of growth, they press into 

 the water-courses 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, and may then 

 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 medullary ray 

 cells. The other cells, or those of the middle layers of the medullary 

 bands, on the other hand, are called horizontal medullary ray 

 cells ; they are narrower and more elongated radially. These have, 

 moreover, no especial connection with the tracheal elements, but are 

 designed for conducting and storing assimilated matter. In the medul- 

 lary rays of the Lime (Fig. 147), although this specialisation of the cells 

 is not so evident as in many other cases, the marginal cells of the medul- 

 lary rays are, nevertheless, particularly noticeable, as they alone have 

 bordered pits on the sides toward the vessels (g), and are also wider 

 than the other cells of the inner rows. 



Within the bast zone the medullary rays are also distinguished as 

 CORTICAL RAYS, and in the bast of Dicotyledons they have a simpler 

 structure than in the wood. It is evident, not only from the con- 

 nection existing between the cells of the medullary rays and the bast 

 parenchyma, but also from the relations exhibited in Dicotyledons 

 between the medullary ray cells and the companion cells of the 

 sieve-tubes, that the function of the cortical rays is to take up the 

 substances passing down the bast strands. For not only is the bast 

 parenchyma in communication with the cells of the medullary rays by 

 means of bordered pits, but the companion cells are so disposed on the 

 sides of the sieve-tubes as more surely to come in contact with the 

 medullary rays. 



In the Pine and other Abictineae, whose bast parenchyma is devoid of cells 

 functioning as conductors of albuminous matter, their place is taken in this respect 

 by rows of medullary ray cells (Fig. 141, em). These maintain an intimate 

 connection with the sieve-tubes by means of sieve-pits. They lose their contents 

 in the same manner as the sieve-tubes, and, like them, become compressed and 



