136 



BOTANY 



disorganised. On the other hand, the cells of the cortical rays, which contain 

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

 between the compressed sieve-tubes, continue living for years. 



The division of labour within the medullary rays of the Gynmosperms and 

 Dicotyledons is so well carried out, that only the rows of elongated, conducting 

 cells are accompanied by intercellular air-spaces. When the walls of such cells are 

 much thickened, they are pierced with pits which open into the intercellular air- 

 passages, and so facilitate the interchange of gases. 



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



from tangential than from radial sections. 

 In such tangential sections the medullary 

 rays appear spindle-shaped (Figs. 141, 

 148). "With few exceptions, as in the 

 Oak and Beech, the medullary rays are 

 rarely of more than limited dimensions. 

 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 rays. These radial 

 resin -duets are in communication with 

 the vertical duets. It is due to this fact 

 that such a large amount of resin exudes 

 from wounds in Pine or Fir trees. 



The koots of Gymnosperms and 

 Dicotyledons, in which the stems 

 increase in thickness, also show a 

 similar growth in thickness. 

 Whenever secondary growth begins 

 in a root with its xylem and 

 phloem ' strands alternating with 

 each other (Figs, 120, 12C), areas of 

 cambium arise on the inside of the phloem strands, through the 

 division of the fundamental tissue ; these give off wood elements 

 towards the centre of the root, and bast towards the periphery. These 

 cambium areas soon meet in the pericycle, just in front of the xylem 

 strands, and so form a complete zone of meristematic tissue. In Fig. 

 150, 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 a simple ring. In front of the primary vascular strands (/), 



o. 150. — Diagrammatic representation of the 

 growth in thickness of a dicotyledonous 

 root, pr, Primary cortex ; c, cambium 

 ring ; g', primary vascular strand ; s', 

 primary phloem strand ; jj, pericycle ; e, 

 endodermis; g", secondary wood; s", second- 

 ary bast ; L; periderm. 



