GROWTH. 369 



Wiesner gives the following account of the growth in length of the 

 epicotyl of Phaseolus multiflorus from its first development to its ma- 

 turity. As it exists in the seed it is short (about i mm.) and straight. 

 When it first begins to grow in length, its growth is uniform but slow, 

 and longitudinal cell-division is very active. It then begins to grow more 

 rapidly and becomes curved by hyponasty ; at this stage cell-division is 

 less active. As growth proceeds it becomes more rapid, and, in con- 

 sequence of epinasty in its lower part, the epicotyl exhibits undulating 

 nutation, cell-division still going on. Finally the epicotyl becomes 

 straight and cell-division ceases : at this stage the rapidity of growth in 

 length is the greatest, but it gradually diminishes until growth ceases 

 altogether. 



It might be suggested that the nutations of hypocotyls 

 and epicotyls are induced by the action of gravity, that is, 

 that they are not spontaneous; but the investigations of Sachs, 

 Wortmann, and Vochting, have shewn that the nutations are 

 not dependent upon gravitation, for they are exhibited by 

 seedlings which are made to revolve slowly round a horizontal 

 axis, so that the action of gravity is uniform on all sides. 



Finally, with regard to the cause of the grand period. We 

 have already (p. 292) laid stress on the dependence of growth 

 upon destructive metabolism, and it has also been pointed 

 out (p. 196) that in growing organs destructive metabolism, as 

 estimated by respiration, is very active. The grand period 

 of growth is -probably to be regarded as the expression of a 

 corresponding grand period of destructive metabolism in the 

 growing organ. Mayer has in fact found that the grand 

 curve of growth in seedlings runs nearly parallel with the 

 curve of the absorption of oxygen by them. 



The influence of external conditions upon growing organs 

 will be considered in the next two or three lectures. 



BIBLIOGRAPHY. 



Sachs ; Text-book of Botany, 2nd English edition, 1882, p. 851. 



The Mechanics of Growth. 



Sorauer ; Bot. Zeitg., 1873. 



de Vries ; Untersuch. lib. d. mechanischen Ursachen der Zell- 



streckung, 1877, p. 54. 

 Strasburger ; Bau und Wachsthum der Zellhaute, 1882, p. 194. 



V. 24 



