352 B. ORCHARD 



The cause of the after-effect on E is still not clear; it is too large to be 

 accounted for by the effect of L on E (Watson, 1958). Drought may permit 

 something to be stored which later promotes growth or the rapid expansion 

 of the plant when the moisture content is increased may provide a sink for 

 assimilates and so promote assimilation. Gates (1955) suggested that tomato 

 plants become more juvenile as a result of wilting and so reversed the 

 general trend for growth rates to fall with age, but this explanation does not 

 apply to sugar beet in which the trend is small (Thorne, i960). Gates (1957) 

 later suggested that changes in phosphorus metabolism were important. 

 The after-effect of drought on E implies that E is at least partially restricted 

 by internal factors and that it should be possible to increase crop yield by 

 increasing E. 



The decrease in growth rate during drought (the direct effect) was only 

 influenced by weather when the plants were small (L< i), although the 

 effect itself was observed in the larger plants. The weather factor almost 

 certainly operated through transpiration, which controls both the time 

 required to remove the Treely available' water from the root zone and 

 the proportion of the soil moisture which is freely available (Bierhuizen, 

 1959; Kuiper and Bierhuizen, 1959). The effect of plant size is not 

 unexpected considering the known interrelations between root growth, 

 transpiration and water supply (Slatyer, i960). Water moves very slowly 

 in unsaturated soils and root extension is essential to the maintenance of 

 the water supply. Once the demand exceeds this supply, the plant turgor 

 and hence root growth wiU fall, further restricting the supply. The larger 

 the active root system and the greater the initial soil moisture content, the 

 greater the water demand the system can sustain. 



In 1959, treatment B decreased both leaf area and dry matter production, 

 but in i960 the same treatment appHed to plants with a similar number of 

 leaves had no effect on dry matter production and only a small effect on 

 leaf area. In 1959 transpiration was evidently rapid enough to exceed the 

 critical demand and severe water stress developed. In i960, transpiration was 

 much slower and did not exceed the critical rate. 



The significant positive correlation between plant moisture content and 

 leaf area on the first sampling occasion in Exp. 3 indicates that the high initial 

 variabihty of the plots was due to differences in soil moisture (remaining 

 from the previous experiment). These appeared to interfere with the early 

 growth of the plants and may have influenced the moisture content at the 

 first sampling indirectly, either by decreasing the amount of stored water 

 in the soil and so slightly hastening the onset of drought, or by affecting the 

 composition of the plant. The significant negative correlation between 



