142 



BOTANY 



rays. These radial resin-ducts 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 Gymnosperms and 

 Dicotyledons. The ROOTS of Gymnosperms and Dicotyledons, in 

 which the stems increase in thickness, show a similar GROWTH IN 



THICKNESS ( 12S ). When secondary 

 growth begins in a root with its 

 xylem and phloem strands alter- 

 nating with each other (Figs. 1 24, 

 131), layers of cambium arise on 

 the inside of the phloem strands, 

 through the division of the funda- 

 mental 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. 151 A, this 

 process is diagrammatically repre- 

 sented. 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 medul- 

 lary rays which lead inwards to the 

 strands of primary xylem (Fig. 151 



Fio. 151. Diagrammatic representation of the B). A crOSS-SCCtion of SUch a rOOt, 



rowth in thickness of a dicotyledonous in which the secO iidary growth has 

 root, pr, Primary cortex ; f, cambium 



ring; g ; primary vascular strand; , continued for sonieyears,can scarcely 



primary phloem strand; j, pericycle; c, De distinguished from a Cl'OSS-section 



of a stem. By careful examina- 

 tion, however, the presence of prim- 

 ary xylem in the centre of the root can be discovered, and its nature 

 thus determined. The wood of the root is also more porous than 

 in the stem, and bears a close resemblance to spring-wood. On 

 account of this lack of differentiation in the wood, the annual rings 

 of growth are less distinctly defined in roots than in steins. 



In the root and its branches, as in the stem, the annual zones of 

 secondary growth become less numerous on approaching the growing 

 points (Fig. 140). 



Anomalous forms of Growth in Thickness ( 1U9 1. Extraordinary <lc\ i;itii>ns from 

 the usual type of secondary ^rowi li arc aH'onli-d l>y -i>mr .-inns and routs <>f ( Jyiiino- 



