386 PHYSIOLOGY OF GROWTH. 



investigation is not very considerable. In the next it is already 

 more active. The maximum growth takes place in the third 

 or fourth zone ; in the following ones the rate of growth 

 again diminishes. If we continue the observations for some time, 

 it will be found that the growth soon ceases in the older zones, 

 while the maximum of growth is no longer situated in the third 

 or fourth, bat in a younger zone. Still later the rate of growth 

 in these latter partial zones in turn diminishes. 2 



In order to prove the existence of the grand period in the 

 growth of leaves, we grow cucumber or tobacco plants in large 

 flower pots, and, when a few leaves have unfolded, place over 

 them large bell-glasses, keeping them in a place where the tem- 

 perature is as constant as may be, e.g. in a room with a north 

 aspect. The research material is left exposed to the light. Near 

 the base of the blade of some of the young leaves we place a 

 dot of ink to serve as a mark. Every day we measure with a 

 millimetre scale the distance between the tip of the leaf and the 

 dot at its base. Naturally the temperature is always to be 

 carefully noted. During the months of May and June I followed 

 carefully the growth in length of leaves of Aristolochia Sipho 

 growing in the open air, and found at first, when the temperature 

 was fairly constant, that the phenomenon of the grand period 

 of growth was very clearly exhibited. The growth during 

 the twenty-four hours was at first only 5 mm., then 7 mm., 

 later on 10 mm. Later still, owing to considerable variations 

 in the temperature, great irregularities became observable in the 

 growth of the leaves, but it is nevertheless instructive to repeat 

 such observations, since they show us how important it is, in 

 studying the grand period of growth in plant structures, not 

 to leave out of consideration, for a moment, external influences 

 affecting their growth. 3 



In order to determine the causes of the grand periods of entire 

 organs, we must, as I have emphasised in my Lehrbuch der Pflan- 

 zenphysiologie, p. 249, explain the variations exhibited during 

 development in the rate of growth of the separate partial zones 

 of the structure. This is done, as far as at present appears 

 possible, in 147, so that we must here refer the student to the 

 account there given. At the commencement of growth in an 

 entire organ, we have at first only the sum of few and insig- 

 nificant increments of growth, later more and larger ones, till 

 finally the increments of growth again become inconsiderable. 



