50 



PEACTICAL BOTANY 



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, ironweeds, 

 hemp, giant ragweed, and 

 so on, growth in thick- 

 ness goes on tliroughout 

 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 

 iirner cells of the cam- 

 bium, in a similar way 

 and to a still greater 

 extent, form new wood, 

 and thus tlie stem goes 

 on increasing in thick. 

 ness. But in sueh plants 

 as thc)se just mentioned 

 the activity of the cam- 

 bium is strictly limited. 

 After it has given rise 

 to a certam 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 



Tig 35 Cio&s sec tion of a stit k of oak wood 



m, medullaiy rays, running from bark U> pith', 

 r, "annual rings"; b, lumndaries bt't^\'een 

 "" rings," ]ii)rous from iireseiire of many ducts ; 

 (', interior fibrous layers of dead liai-k ; pi, hard 

 plates of dead bark, splitting away from each 

 other but attached to bark beneath. Reduced 



