SECT, ii PHYSIOLOGY 257 



region extends over only about one centimetre of the extreme tip, often indeed over 

 only centimetre, while all the rest of the root has already completed its growth in 

 length. The length of the growing region is influenced by such external conditions 

 as mechanical hindrance, cold, warmth, dryness, etc., and exhibits consequent 

 variations. That the grand period is exhibited by this short growing region may 

 be made clear by marking oft' with india-ink, near the tip of a root, narrow zones 

 of equal width, which would thus also be made up of cells of nearly equal size. In 

 Fig. 217 /, is shown a germinating Bean (Vicia Faba), whose root-tip has been 

 marked in this way ; Fig. 217 //, represents the same root after twenty-two hours 

 of growth. The marks have become separated by the elongation of the different 

 zones, but in different degrees, according to their position. The greatest elonga- 

 tion is shown by the transverse zone 3 ; from there the growth in length decreases 

 towards the younger zones (2 and 1), as well as towards the older (4 to 10). This 

 peculiar distribution of growth is but the result of the grand periods of growth of 

 the cells in zones of different ages ( 68 ). In the millimetre-broad zones of a root of 

 Vicia Faba SACHS found, after twenty-four hours, that the increase in growth, 

 expressed in tenth-millimetres, was as follows : 



Zones: I., II., III., IV., V., VI., VII., VIII., IX., X., XI. 

 Increase : 15, 58, 82, 35, 16, 13, 5, 3, 2, i, o. 



The elongating region in shoot axes is generally much longer than in roots, and 

 usually extends over several centimetres, in special cases even over 50 or more 

 centimetres. The distribution of the increase corresponds in stems, as in roots, with 

 the grand periods of, growth of the cells. Even by INTERCALARY GROWTH, where 

 the region of elongation is not confined to the apex but occurs in any part of the 

 organ, generally at its base (leaves and flower-stalks of many Monocotyledons), 

 grand periods of growth are also apparent. A shoot of Phascolus multiflor'ns which 

 was divided, from the tip downwards, into transverse zones 3*5 mm. broad, showed 

 in forty hours, according to SACHS, 



in zones : I., II., III., IV., V., VI., VII., VIII., IX., X., XL, XII. 

 an increase of 20, 25, 45, 65, 55, 30, 18, 10, 10, 5, 5, 5 

 tenth-millimetres. 



This periodicity in the growth in length occurs even when the external influences 

 affecting growth remain constant, and is determined by internal causes alone. 



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

 MIYAKE, in the scapes of the Dandelion, 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 Linana cymbalaria, and Arachis hypogaca (p. 285), and floral envelopes which 

 later protect the fruits in other cases). 



External Influences upon Growth ( 69 ). External factors often 

 take an active part in the process of elongation, either as retarding 

 or accelerating influences. As growth is itself a vital action, it is 

 affected by any stimulus acting upon the protoplasm ; on the other 

 hand, as it is also a physical function, it is modified by purely physical 

 influences. Growth is particularly dependent upon temperature, 

 light, moisture, the supply of oxygen and other substances, and the 

 existence of internal pressure and tension. It is also influenced by 

 injuries. 



The INFLUENCE OF TEMPERATURE is manifested by the complete 



s 



