382 PHYSIOLOGY OF GROWTH. 



mm. per hour, and the experiments may be continued for a long 

 time (for further experiments after this method see below). 



If it is desired to determine the growth in short intervals of 

 time of the stems of seedlings {e.g. in the hypocotyl of Lepidium), 

 fine ink lines must be painted on them with Indian ink. Soon after 

 their development has begun, the seedlings are fixed in small 

 glasses by means of cotton wool, in such a way that their roots dip 

 into water, or they may be cultivated in small clay cylinders con- 

 taining sawdust. They are placed under a bell-glass and subjected 

 to slow rotation on the clinostat. Sharp angles or projections 

 of the ink-marks serve as points of reference for the measure- 

 ments. 



1 See Sachs, Arbeiten d. hot. Instit. in Wiirzbury, Bd. 1, p. 113. 



2 Pfeffer, Handbuch d. PJlanzenphysiologie, Bd. 1, p. 86. 



154. The Grand Period of Growth. 



It is a fact of fundamental physiological significance that all 

 growing plant structures (roots, stems, leaves, etc.), even under 

 constant external conditions, do not experience the same amounts 

 of growth in equal successive intervals of time. Every part at the 

 commencement of its development grows slowly ; gradually its rate 

 of growth becomes more rapid, attains a maximum, then becomes 

 more slow again, and finally growth completely ceases. To prove 

 this in a general way first of all, it is sufficient to soak a few pea 

 seeds, and lay them in a crystallising glass containing enough 

 water to half cover them. Or by means of cotton wool we fix a 

 pea or bean seed which has just germinated in the hole of a cork 

 which closes a glass vessel containing water, so that the root of the 

 seedling grows downwards in the water. Germination is allowed 

 to proceed in the dark, at as constant a temperature as possible 

 (say 20 C.), and we ascertain every day at a particular hour the 

 length of the roots. It is found that the growth of each root is at 

 first comparatively slight, gradually becomes more considerable, 

 sooner or later (in my experiments, made at a temperature of 

 16 C., on the ninth day) attains a maximum, and then gradually 

 falls off again. 



We soak good seeds of Pisum, Phaseolus, or Vicia Faba, for 

 twenty-four hours in water. The seeds are then put into moist saw- 

 dust, which has previously been rubbed down between the flat hands, 

 and filled into large wooden boxes or flower pots to form a loose seed 



