SECT. I 



MORPHOLOGY 



141 



elements, but are designed for conducting and storing assimilated substances. 

 Within the bast xone 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 companion 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. 143 em}. These maintain an intimate connec- 

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



Fio. 150. Portion of a transverse section of the bast of Tilin ulmifnli". v. Sieve-tubes ; v*, sieve- 

 plate ; c, companion cells; k, cells of bast parenchyma containing crystals ; ;>, bast pui-en- 

 chyma ; f. bast fibres ; /, medullary ray. (x :>W.) 



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

 organised. On the other hand, the cells of the medullary rays, which contain 

 starch, like the similar cells of the bast parenchyma, increase in size, and continue 

 living for years. 



The width and height of the medullary rays may be more easily determined 

 from tangential than from radial sections. In such tangential sections the medul- 

 lary rays appear spindle-shaped (Figs. 144, 119). With few exceptions, as in the 

 Oak and Beech, the medullary rays are of relatively small size. The Oak, in 

 addition to numerous small medullary rays, has other larger rays which may be as 

 much as a millimetre broad and a decimetre high. In the Poplar, Willow, and 

 Box the medullary rays are so extremely small that they are scarcely visible, even 

 with the aid of a magnifying glass. The height of the broad primary rays of many 

 lianes, on the other hand, may be equal to that of a whole internode. In certain 

 Conifers, resin-ducts occur not only in the wood, but also in the broader medullary 



