156 



BOTANY 



PAKT I 



by them and in other cases in contact with them on one side 

 (Fig. ISQp). 



The wood parenchyma surrounds the vessel in Acacia, etc. ; it forms tangential 

 bands in which the vessels are embedded or with which they are in contact in 



Walnut, Chestnut, Oak, etc. ; in some cases 

 it is limited to the outer side of the annual 

 ring. 



The MEDULLARY RAYS (FigS. 178 



pm, sm; 180, 181 r) resemble those of 

 the Gymnosperms in being radially- 

 placed bands of tissue, of greater or 

 less vertical height, and one or a 

 number of cells in breadth ; they may 

 be branched or unbranched (Fig. 182 

 tm, sm). They are continuous across 

 the cambium into the bast (Fig. 178). 

 The vascular strands are in contact 

 with them at places. The parenchyma 

 of the medullary rays thus connects 

 the parenchyma of the bast with that 

 of the wood, and unites all the living 

 tissue of the stem and root into a single 

 system. Assimilated material moving 

 downwards in the bast can thus pass 

 radially into the wood and be carried 

 in this for some distance upwards or 

 downwards, to be stored as starch in 

 the living parenchymatous cells. The 

 intercellular spaces, which accompany 

 the medullary rays and the strands 



of TUia uimifoiia. m, Pitted vessel ; of wood parenchyma, allow of the 

 t, spiral tracheides ; P) wood paren- gaseous exchanges necessary for the 



chynia ; I, wood-fibres ; r, medullary ? ' . /. . , n 



rays, (x 160. After SCHENCK.) living elements of the WOOd. 



The intervals between the strands 



of vessels and of parenchyma and the medullary rays are occupied 

 by strands of wood-fibres (sclerenchyma). 



The height and breadth of the medullary rays are most readily seen when they 

 are cut across in tangential longitudinal sections of the stem ; the rays then appear 

 spindle-shaped (Fig. 181 r). In most woods their size varies only within narrow 

 limits, but in others, such as the Oak and the Beech, the range is greater. In the 

 Oak there are medullary rays which are 1 mm. wide and 1 dm. high, while 

 numerous small rays occur between these. In the Poplar, Willow, and Box all 

 the rays are so small as to be with difficulty distinguished even with the aid of 

 a lens. In some lianes (e.g. Aristolochia] the primary rays are particularly wide 

 and high, and may extend for the length of a whole internode. 



