DIV. II 



PHYSIOLOGY 



281 



successive days it is found that it grows at first slowly, then rapidly, and then 

 again slowly. In other words, every division of the growing zone exhibits the 

 grand period of growth. The millimetre zones marked off from the apex are 

 thus in different stages of their grand periods ; 

 the two first are on the ascending side of the 

 curve, 3 and 4 are at the summit, and the 

 others are on the descending slope of the curve. 

 Other organs give corresponding results. 



Distinct periods of growth separated by an 

 interval of time occur in the scapes of the Dande- 

 lion, the first period in relation to the develop- 

 ment of the flowers, the second to that of the 

 fruits. A similar behaviour is found in other 

 organs whose function after a time becomes 

 altered (flower or fruit stalks in Linaria cymba- 

 laria, and Arachis hypogaea). 



Rate of Growth. From the fact that 

 in different organs zones of different 

 length are in a growing condition, it 

 follows that such results as to the total 

 growth of an organ as were described on 

 p. 279 do not give the true rate of growth, 

 i.e. the growth of a unit of length in unit 

 time. Thus in the shoots of the Bamboo 

 the growing zone is many centimetres 

 long, while in Botrytis it is only 0'02 mm. 

 in length. While Bambusa shows twice 

 as much growth per minute as Botrytis 



does, itS rate Of growth is really much FIG. 256. Unequal growth of different 



less. To express the rapidity of growth 



it is necessary to express the elongation 



per minute as a percentage of the growing 



zone. This gives a rapidity of growth 



of 83 per cent in Botrytis, and of only 



1*27 per cent in Bambusa. The maximum 



growth observed is 220 per cent in some 



pollen tubes, while some shoots which are still clearly growing have a 



rate of only 0'5 per cent. 



Size of the Plant. We can only determine the definite elongation 

 of a part of the plant when, in addition to the rate of growth and the 

 length of the growing region, the duration of growth is known. The 

 size of the plant, which, as is well known, depends in various ways on 

 external conditions and yet is a specific character, is determined by 

 variations in these factors. A definite size belongs to the specific pro- 

 perties of an organism just as much as the form of its leaves, etc. ; 

 further, the whole organisation of the plant is such that it involves a 

 particular size. The stems of twining plants are particularly long, 



o J 



regions of the root-tip of Vina, Faba. 

 I, The root-tip divided by marking 

 with india-ink into 10 zones, each 

 1 mm. long. //, The same root after 

 twenty -two hours ; by the unequal 

 growth of the different zones the 

 lines have become separated by un- 

 equal distances. (After SACHS.) 



