PHYSIOLOGY 



485 



the growing apex is being constantly carried forward by the elongation 

 of the shoot, the path traced by the apex will be either a spiral or a 

 zigzag. These movements affect only the apex, and the stem does 

 not necessarily 

 become twisted, 

 although where 

 there is torsion 

 of the stem it 

 follows the direc- 

 tion of the nuta- 

 ting apex. 



The length of 

 the nutating apex 

 may be consider- 

 able (about 40 cm. 

 in the Hop), and 

 the time required 

 for a complete 

 revolution varies 

 greatly, even in 

 the same plant. 

 Thus in Akebia 

 quinata, the long- 

 est time (accord- 

 ing to Darwin) 

 necessary for a 

 complete revolu- 

 tion of the apex 

 was four hours, 

 the shortest, one 

 hour and forty- 

 five minutes. 



The uncoiling 

 or unfolding of 



young leaves and of such coiled inflorescences as those of various 

 Borraginaceae (Myosotis, Heliotrope), and the opening and closing 

 of flowers, are also forms of movement due to growth. 



Rate of Elongation. The rapidity of growth in elongating organs 

 is sometimes extraordinarily great. Many climbing plants e.g. 

 Hop, Cobaea will show an elongation of the shoot amounting to 

 10 to 15 centimetres or more in twenty-four hours, and this is far 

 surpassed by the growth of many tropical plants, like the Bamboos, 

 Agaves, etc. In the Bamboos the elongation may amount to 90 cen- 

 timetres or more. The whole growth of a Bamboo, 30 to 40 metres 

 in height, takes place in a single growing period. Drude found that 



FIG. 458. Twining stems. A, Hop. B, Morning-glory. 

 (After BATLEY.) 



