PLANT PETSIOLOQT. 



89 



growing period, while others retain their power of growth 

 for long periods. The woody stem of an ordinary dicoty- 

 ledonous shrub or tree, for example, consists of masses of 

 different kinds of cells which soon lose their power of 

 growth; thus the wood-cells, vessels, and even the paren- 

 chymatous cells of the wood, pith, and bark are soon 

 incapable of growth in size, and retain but little longer the 

 power of growth in thickness of the wall. In the same 

 stem certain other cells (lying between the wood and bark, 

 and commonly known as the cambiam) retain their grow- 

 ing power for many months, and it is these which enable 

 the plant to increase its diameter year by year. 



158. Growth in Length. — Since most cells have a 

 limited period of growth it follows that in the growth of 



...b 



a 

 \ / 9 c 



Fig. 49. Fio. 50. 



Fig. 49.— Grovrth of the root. A, root marked with India ink. B, the 

 same root after further growth. 



Fig. 50.— Instrument (auxanometer) for measuring growth of stems. 

 a, a delicately constructed index balanced by the weight b ; c, weight on 

 thread which passes over the pulley to the plant ; d, graduated arc : one- 

 tenth natural size. 



an axis each part retains its power of elongating for a short 

 time only. In roots the elongation of cells and, as a con- 

 sequence, of the root itself, is confined to the terminal por- 

 tion (Fig. 49). Many stems retain their power of growth 



