, u BOTANY OF THE LIVING PLANT 



a long nail tends to bend when it is being driven into hard 

 wood. It will be seen later thai the elongation of the cells of shoot 



.,n«l root i- subject to control by the apical tissues. 

 The elongation of cells in the zone of most active growth of a 

 m or a i liable to be proceeding, at any moment, at varying 



rates on different sides of the organ. 

 In this event the apical part of the 

 organ will tend to curve away from 

 the side of greatest growth. If the 

 tendency towards most rapid elonga- 

 tion moves regularly round the stem, 

 the tip will exhibit a revolving move- 

 ment, and will pursue a spiral course 

 through space. The shoot-apices of 

 many plants do exhibit such a revolving 

 movement or circumnutation, as was 

 first discovered by Darwin. But the 

 movements are not readily discernible 

 owing to their slowness and small 

 amplitude. Darwin devised a method 

 of magnifying the movements and of 

 projecting a record of them on to a 

 plane surface (Fig. 90). It will be seen 

 that the movements are slow and some- 

 what irregular, although there is a rough 

 revolution round a central point. These 



Fig. 89. r 



Localisation of growth sear to the nutatory movements are of much greater 



root-tii) of I'icia Faba. In I. the root •■• i 11 • 1 • t. 



baa been marked with 10 zones 1 mm. amplitude and have special significance 



■part In II. the same root after • . 1 1 r . ■ ■ 1 1 • -1 



rhe lines nearer to the tip are in the shoots ot twining plants, and in the 

 ESfojn!eei?moBt **£? there. 6 ^ftS tendrils of climbers (pp. 215, 2 1 6). Nut- 



^trasburger.) . i r j • 



atory movements are also lound in roots. 

 During the enlargement of the cells, or subsequent to it, structural 

 • litfcrcnti.it ion sets in: this leads to the production of vascular 

 elements from some cells, photosynthetic tissue from others; and so 

 on, according to their position within the growing organ (Chapter II.). 

 Tin- mark- the last stage in the formation of the cell : when completed 

 the cell is mature and normally it does not exhibit further growth. 



{b) Measurement of Growth. 



There are 1 attributes of the growing plant, or plant-organ, 



that can be mad.- the basis of observation when an estimate of growth 



. 



