SECT. II 



PHYSIOLOGY 



259 



Still farther from the growing point the cells are full grown. The 

 length and position of the growing zone depends mainly on the 

 nature of the organ, and is little ati'ected l)y external factors. In 

 roots it is situated at the tip and occupies a length of 5 to 10 mm. 

 In shoots it has generally the same situation, but extends for a length 

 of from several centimetres to half a metre. In many shoots, especially 

 those divided into nodes and internodes, there are a number of zones 

 of growth separated by fully grown zones. 

 This is termed intercalary growth and is 

 beautifully shown, for example, in the 

 haulms of grasses, where a growing zone 

 is found at the l)ase of each internode. 

 At the bases of many leaves also, especially 

 of Monocotyledons, an intercalary grow- 

 ing zone is found. 



The distribution of growth in any 

 member of the plant is ascertained by 

 periodically measuring the distance be- 

 tween certain natural or artificial marks. 



Thus, for example, the tip of tlie root in Fig. 

 210, I, is marked with lines of india-ink at 

 intervals of 1 mm. The marks start from the 

 growing point of the root (0) just behind the 

 root-cap. Twenty-two hours later the marks 

 had been separated from one another as is shown 

 in Fig. 210, II. Tlie elongation has been unequal 

 in the different zones ; at the upper and lower 

 ends of the marked region it diminishes and 

 thus leads to the fully grown region on the one 

 hand and the embryonal region at the tip on the 

 other. Between these and nearer to the apical 

 end is a zone M'liere the maximal growth has 

 taken place. If the growtli of one transverse 

 zone such as that between and 1 is followed 

 on successive days it is found that it grows at 

 first slowly, then rapidly and then again slowly. 

 In otlier words every division of the growing 



oil 



Fill. 210. — Unequal growth of different 

 regions of the root-tip of Vicia Faha. 

 I, tlie root-tip divided by marking 

 with iudia-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.) 



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 tlie descending slope of the curve. Other organs give 



corresponding results. 



Distinct periods of growth separated by an interval of time occur, according to 

 MiYAKE, in tlie scapes of the Dandelion, tlie first period in relation to the develop- 

 ment of tlie 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 exjmbalaria, and AracJits hypogaca floral envelopes which later protect 

 the fruits and perliaps some ovaries). 



