540 LECTURE XXXII. 



ascertain the elongation or growth which has taken place in the same intervals, 

 subtracting each time the preceding length from the one subsequently measured, and 

 denoting the remainder as growth. It is necessary to take care, however, that the 

 temperature remains constant during the periods of observation, or, since this is very 

 difficult to attain, that it varies within equal Hmits, and it is also well to exclude 

 the light from the object under observation, because this also influences the rapidity 

 of growth; since the point is to observe .the process of growth under constant 

 external conditions. Thus, I found in the case of a transverse disc, originally i mm. 

 long, above the growing-point of the primary root of a seedling of Vict'a Faba grow- 

 ing in moist air, with a daily recurring variation of temperature of i8°-2i'5°C., the 

 following alterations in a period of 24 hours : — 



Growth, 



ist day 1-8 mm. 



2nd „ 3-7 „ 



3rd „ iV-5 .. 



4th „ i6-5 „ 



5th „ I7-0 » 



6th „ 14-5 • „ 



7th , 7-0 „ 



8th ., o-o ,, 



Total . . 78-0 mm. 



A similar transverse zone of the internode of a shoot-axis, marked off by trans- 

 verse lines on the youngest part that can be manipulated, behaves also much in 

 the same way as that on a root. Thus, for example, a transverse zone, 3-5 mm 

 long, of the first internode of a seedling of Phaseolus mullißorus, was marked bylines, 

 and the following elongations or growths observed in each period of 24 hours — the daily 

 variation of temperature being from 10-2°— 11° R. 



Growth. 



ist day 1-2 mm. 



2nd ,, 1-5 „ 



3rd , 2-5 „ 



4th , 5-5 ., 



5th , 7-0 ,, 



6th 9-0 ,, 



7th 140 ., 



8th lo-o „ 



9th ,, 7-0 „ 



lOth , 2-0 ,, 



Total . . 59-7 mm. 



Numerous other measurements leave no doubt that the growth in length of 

 transverse discs of leaves and other organs also runs its course in the same way, and 

 from observations of another kind we may conclude that in organs which consist 

 only of simple vesicles not divided up by cell-walls, the behaviour is the same. 



Every small transverse zone of a growing organ thus exhibits a periodic 



