no JIRICATSKY 



estimation but largely also to the heterogeneity of the leaf-blade (Slavik, 

 1959)- Another very important source of dispersion in young leaves lies 

 in the anatomical structure of these leaves, particularly the high number 

 of large nerves, which cannot be avoided in young leaves and which 

 might considerably affect the values obtained. 



So far attention was devoted solely to problems of method. Let us 

 consider the results from a general point of view. As was said above, the 

 uptake of water by a detached young leaf or tissue is caused by a tendency 

 to saturate the actual water deficit and secondly by the requirement for 

 water used for extension growth. On the basis of these findings it was 

 possible to distinguish between the actual water deficit and the 'growth'- 

 WSD produced in the course of time. We are not dealing here purely with 

 a problem of method; if the water deficit is considered as a dynamic feature 

 expressed by water requirement then the growing tissues wiU really possess 

 a sort of potential water deficit. The 'growth'-WSD thus cannot be 

 separated from the actual WSD in any strict fashion even if their distinction 

 is quite clear. This growth-WSD is shown very markedly in experiment 

 no. 2 where, during a complete cut-off of water supply to a saturated tissue, 

 this tissue produces a growth-WSD, which, of course, becomes at once an 

 actual WSD. After restoring the water supply this actual WSD is satur- 

 ated. A prerequisite of this compensation is the following : the true actual 

 water deficit, i.e. the sum of the WSD caused by wilting (decrease in 

 weight) which can be hardly avoided methodologically, and of the 

 growth-WSD must not reach such level and must not act for such a long 

 time as to cause inhibition of extension growth. 



On the basis of these facts some measurements of the actual WSD were 

 carried out on plants during decreasing soil moisture. It was the aim of 

 these experiments to employ the proposed method for obtaining some 

 fundamental data on the wilting of plants in situ, which could serve as a 

 basis for more detailed investigation of the dynamics of wilting. The results 

 support quite unequivocally the view about the preference of young leaves 

 during wilting of the whole plant. It was not possible, however, to demon- 

 strate quite unambiguously the differences in the water deficit at the 

 beginning of wilting, i.e. at WSD between 5 and 8%. It appears that within 

 this range young leaves possess actually a sHghtly higher deficit than mature 

 leaves which may be expected in view of the greater transpiration intensity 

 of young leaves (Slavik, 1956; Halevy, 1956). It cannot be demonstrated 

 quite conclusively, however, whether this, even if small difference, is not 

 due to a very intense extension growth of the tissue at the beginning of 

 estimation. With the possibility of the growth intensity being decreased 



