GROWTH UNDER CONSTANT EXTERNAL CONDITIONS 



IT 



of growth, as determined by Errera 1 , is graphically represented in Fig. 4. 

 The retardation and stoppage of growth at the end of the first day occur 

 when the plant's energies are directed towards the production of the 

 sporangium. After this the stalk rapidly increases in length, until towards 

 the end of its development the hourly increments of growth slowly 

 decrease. The same applies to the sporangiophores of Mucor mucedo and 

 of many other Mucorini, whereas that of Pilobulus crystallinus ceases to 

 grow as soon as the sporangium is formed. In this case the grand period 

 of growth is represented by the first part of the curve in Fig. 4. In the 

 absence of light the sporangia of Pilobtihts are not produced, and the 

 sporangiophore continues its apical growth until long sterile filaments 

 are formed 2 . 



mm 



3,5 



3.0 



2.5 



2fl 



r.s 



i.o 



0.5 



6 Hours. 



First day. 



Second day. 



Third day. Fourth day. 



FIG. 4. Curve of grand period of growth of sporangiophpre of Phycomyces nitens during development 

 under favourable and constant external conditions. The ordinates give the hourly increments of growth in 

 millimetres. 



The relation between the regions where growth by cell-division and 

 growth by cell-stretching are active, differs in stems as well as in roots, 

 and the total length of the growing region may also vary considerably. 

 In some plants it may be from i to 5 centimetres long, whereas in others, 

 and especially in climbing plants, it may attain a length of from 10 to 50 

 centimetres. The growing zone may include either a small or large 

 number of internodes, of which usually a few, but sometimes not more 

 than one or two, have escaped from the bud and are undergoing active 

 elongation 3 . 



At the end of summer these latter zones are gradually transformed 

 into permanent tissue, and fresh growing zones reappear in spring when 

 the buds unfold. The same change can be produced by mechanically 

 arresting growth. Thus after a root has been imbedded for some time in 



1 Errera, Bot. Ztg., 1884, p. 501 and plate 8. 



2 Grantz, Ueber d. Einfluss des Lichtes auf d. Entwickelung einiger Pilze, 1898, p. 10. (Cf. 

 Sect. 24.) 



3 For details see Askenasy, 1. c. (Neue Methode, &c.), 1878, p. 74. On rhizomes see Stahl, 

 Ber. d. Bot. Ges., 1884, p. 384. 



