144 E.B.OKSBJERG 



differences, the only cause appears to be that the weather of the summer of 

 1955 has influenced the two flushing types in different ways. The check in 

 leader growth of the late-flushing plot continued in the following years 

 and significantly different figures for early compared with late-flushing 

 spruces were for 1955, 33-26; for 1956, 19-9; and for 1957, 21-9 cm. 

 Later the differences diminished and have now disappeared. 



Some observations on visible damage proved that the late-flushing 

 spruces had less resistance to drought. Of 140 plants in every plot, 100 

 standing on bare soil (sand, Hght coloured) and 40 in grass vegetation (sand 

 rich in humus), 2 and 37 respectively died in the late-flushing plot but in 

 the early-flushing plot only and 2. In addition, damage such as distortion 

 of the shoots, death of the younger shoots or parts of them, needle loss, 

 etc. predominantly occurred in late-flushing plants (see Fig. 2). 



In the spring of 1956 it was evident that in Centraljutland only extremely 

 late-flushing spruces flowered. That means that flowering is influenced not 

 only by general temperature effects but also in some cases by a decided 

 drought, and that the progeny of seed collected in 1956 do not represent 

 the whole population of a locality, but only the late-flushing part of this. 



In the summer of 1955 the sprouting period of the late-flushing plants 

 coincided with the culmination of high temperature and drought, whereas 

 the early plants had finished growth and to some extent had lignified the 

 shoots before the conditions became critical. The severe damage and the 

 growth depression in the late-flusliing plot could be presumed to be caused 

 by high transpiration rates of the leafy, soft shoots having a very thin 

 peridermis. An examination of the transpiration situation for one-year-old 

 shoots and sprouting ones showed, however, that in most cases the older 

 had highest values whether these were expressed on the basis of weight or 

 of surface area, and whether of shoot or only of needles. When sprouting 

 shoots are terminating their growth, transpiration seems to increase, 

 probably in connection with the differentiation of stomata (Oksbjerg, 



1961). 



During drought periods the transpiration rate of sprouting shoots will 

 be higher than or equal to that of one-year-old ones, probably because of 

 the fact that the main part of transpiration during drought passes through 

 the cuticule. In the last weeks ofjuly, 1955, 1 found that transpiration from 

 one-year-old shoots had nearly stopped, whereas young shoots, checked in 

 sprouting by the drought, continued to loose water. 



Another part of the explanation of the pronounced difference in drought 

 damage to late- and early-flushing spruces is differences in 'hydrature' in 

 the sense of Walter (193 1). The growth features and related metabohc 



