152 



BOTANY 



PART I 



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

 disorganised. 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. 153, 158). 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 small 

 that they are scarcely visible, even M'ith 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- 

 passages occur not only in the wood, but 

 also in the broader medullary rays. These 

 radial resin-passages are in communication 

 with the vertical ducts. It is due to this 

 fact that such a large amount of resin 

 exudes from wounds in Pine or Fir trees. 



Secondary Thickening- of the 

 Root in Gymnospepms and Dicoty- 

 ledons. — The ROOTS of Gymno- 

 sperms and Dicotyledons, in which 

 the stems increase in thickness as 

 well as those of many herbaceous 

 Dicotyledons, show a similar GROWTH 

 IN THICKNESS (^*"). When second- 



Fio. 160.-Diagrammatic representation of the ary grOWth begins in a rOOt with 

 growth in thiclvness of a dicotyledonous ■, ^ t i i , t t, 



root, pr, Primary cortex; c, cambium its xylem_ and phloem strands alter- 



ring; g', primary vascular strand; s', nating with each Other (FigS. 134, 

 primary phloem strand; p, pericycle; e i^q\ ] .g ^f Cambium arise On 



endoderinis; g", secondary wood ; s, second- i ' • , p i , i t 



ary bast ; t, periderm. the inside of the pluoem strands 



by the division of the fundamental 

 tissue ; these give off wood elements towards the centre of the 

 root, and bast towards the periphery. These cambium layers soon 

 meet in the pericycle, just outside the xylem strands, and so form a 

 complete cambial ring. In Fig. 160 A, this process is diagrammatically 

 represented. As a result of the activity of its cells the cambium ring 

 soon loses its sinuous form and becomes circular. In front of the 

 primary vascular strands (g'), the cambium produces medullary ray 

 tissue, and this constitutes the broadest medullary rays which lead 

 inwards to the strands of primary xylem (Fig. 160 B). A cross- 



