SECT. I 



MORPHOLOGY 



151 



in the spring, at the beginning of the season of growth, they press into the water- 

 courses the products of assimilation, particularly glucose and small quantities of 

 alburainatps, in order that these substances may be transferi'ed in the quickest way 

 to the points of consumption. Accordingly, during the winter and in the begin- 

 ning 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 



Fio. 159. — Portion of a transverse section of the bast of Tllia ulmi.folia. v, Sieve-tubes ; u*, 

 sieve-plate ; c, companion cells ; k, cells of bast parenchyma containing crystals ; pi t>^st 

 parenchyma ; ', bast fibres; r, mertuUary ray. (x 540.) 



MEDULLARY RAY CELLS. The othcr 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 ("i). These have no especial 

 connection with the tracheal elements, but are designed for conducting and 

 storing assimilated substances. Within the bast zone the medullary rays of 

 Dicotyledons have a simpler structure than in the wood. It is evident, not only 

 from the pits between the cells of the medullary rays and the bast parenchyma, but 

 also from the similar relations in Dicotyledons between the medullary ray cells 

 .and the comiDanion cells of the sieve-tubes, that the function of the cells of the 

 medullary rays in the phloem is to take up substances passing down the bast strands. 

 In the Pine and other Abietineae, 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. 152 em). These maintain an intimate 

 connection with the sieve-tubes 1 y means of sieve-pits. They lose their contents 



