GROWTH AND MOVEMENT 



119 



by any means be restored again to their original state. Growth may 

 then be defined as an increase in size accompanied by a redistribution of 

 available material. 



The conditions under which growth is possible are the followinf/ : 



(i) A supply of plastic material in the plant. 



(ii) Opportunity for gaseous interchange. 



(iii) A temperature between certain limits. 



(iv) Turgor in the cells. 



The growth which ensues when these conditions are fulfilled may 

 affect any of the dimensions of the growing part. The most obvious 

 is growth in length, as shown 

 in stems or roots. But 

 growth may also be in thick- 

 ness: or both dimensions 

 may increase together. In 

 the body of the Higher 

 Plant growth is not uniformly 

 carried out in all parts. It is 

 localised. Certain zones or 

 regions of a stem or root 

 may be growing, while other 

 parts do not alter. The dis- 

 tribution of growth in length 

 may be demonstrated in 

 young herbaceous stems or 

 in roots, by measuring off on 

 them by a rule units of length 

 from the apex downwards, 

 and marking them with 

 Indian ink. After a period 

 of a day or so a comparison 

 of these marks with the 

 original scale will show that 

 their distances have in- 

 creased, but that the increase 

 is not uniform. (Fig. 83.) 

 The greatest elongation will 

 be shown in the growing 



, . ^ . , ^ Fig. S3, 



shoot at some pomt consider- -n.e kft-hand tigure shows a growing shoot at the 



^l^U, \ ^1^ i-U i.- 1 -1 i. begiiiniiig, the right-hand figure at the end of the period 



ably below the tip, while it of observation. See Text, (.^fter Errera.) 



