50 



PRACTICAL BOTANY 



-* m 



pulls, but laterally they are supported by the earth. Accordingly 

 it is usual to find young roots with a fibrous central cylinder of 

 , comparatively small di- 



ameter, surrounded by a 

 coating of much weaker 

 tissue (Fig. 34, C). 



47. Limited thicken- 

 ing of annual stems. In 

 stems of large dicoty- 

 ledons which die to the 

 ground every year, such 

 as sunflowers, iron weeds, 

 hemp, giant ragweed, and 

 so on, growth in thick- 

 ness goes on throughout 

 the summer. The outer 

 cells of the cambium con- 

 tinually split up, by the 

 formation of tangential 

 partitions (parallel to 

 the bark), and so form 

 new layers of bark. The 

 inner cells of the cam- 

 bium, in a similar way 

 and to a still greater 

 extent, form new wood, 

 and thus the stem goes 

 on increasing in thick 

 ness. But in such plants 

 as those just mentioned 

 the activity of the cam- 

 bium is strictly limited. 

 After it has given rise 

 to a certain amount of new tissue, growth stops and the stem 

 dies down to the ground. The death of annual stems in the 

 autumn is often thoughtlessly supposed to be due to the 



FIG. 35. Cross section of a stick of oak wood 



m, medullary rays, running from bark to pith ; 

 r, "annual rings"; b, boundaries between 

 " rings," porous from presence of many ducts ; 

 i, interior fibrous layers of dead bark ; pi, hard 

 plates of dead bark, splitting away from each 

 other but attached to bark beneath. Reduced 



