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. 



Tlie conditions under wliich growth is possible arc the following : 



(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 groioth 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 mcrease 

 is not uniform. (Fig. 83.) 

 The greatest elongation will 

 be shown in the growing j__^^ ^^^^ 



shoot at some pomt consider- Chc left-haml figure shows a growing shoot .rl the 



, .. ■ lir;;iiiiiiiig. the right-hand figure at the end of the i^eriod 

 ablv below the tip, wnile it ,,( observation. See Text. (Alter Errera.) 



