HYDRATION, PHOTOSYNTHESIS AND RESPIRATION 229 



The photosynthesis rate was measured in 28 leaves, i.e. in a total of 28 

 pairs of apical and basal parts of the leaf-blade. The leaf chambers were 

 placed as shown in Fig. i by the rectangles. At the beginning of each 

 experiment the stomata were opened photoactively (previous saturation 

 with water took place in the dark) so that after 20 or 30 min the intensity of 

 photosynthesis reached mostly a steady value which was maintained for 

 at least one hour but usually for several hours. The plants were well watered 

 and the conditioned air passing through the leaf chamber had a high 

 humidity so that the water saturation deficit formed was reflected in a 



Fig. I. Localisation of the chambers in photosynthesis determinations (rectangles) and 

 of the disks cut out for respiration measurements (circles). 



decrease of the apparent photosynthesis only after several hours' exposure. 

 The water saturation deficit values at which a marked drop in photo- 

 synthesis could be observed on long exposure varied about the value of 

 10 per cent. For our results, values of photosynthesis were chosen from the 

 initial, completely steady measuring intervals. An important fact should be 

 mentioned here, however, namely that the basal part of the leaf revealed in 

 a separate experimental series a quite regularly higher water saturation 

 deficit than the apical parts. Thus the possible differences in the water 

 deficit could affect the difference in photosynthesis in the reverse direction 

 only, i.e. they could decrease the intensity of photosynthesis in the basal 

 part, thus diminishing (and in no case increasing) the observed difference 

 in the intensity of photosynthesis between the base and the apex. 



The results showed that under the given experimental conditions the 

 intensity of photosynthesis referred to area was on the average 20-2 per 

 cent higher in the basal part of tobacco leaves than in the apical one. The 

 observed average difference d= 2-1 ±0-4. mg COg/dm^hr (the average 



