Growth in Eriogonum nudum 223 



becomes less. Both the minima and maxima of the march fall at the 

 same hours, and the behaviour is attributable to factors identical with 

 those operative for internodes during the period of maximum growth 

 (cf. MacDo.ugal, 191 8, p. 59). 



These behaviours are I believe understandable when the conditions 

 affecting transpiration are considered). The closure of the stomata 

 which occurs on wilting pe^rmits the increase in the net water content 

 of the growing part, and this in turn, the assumption of restitution 

 which, as we have seen, is accomplished rapidly on sunny days, that is, 

 when the most complete wilting and stomatal closure have occurred. 

 But on such days the period of high evaporation persists till the 15 or 

 16 hour so that, although the part has displayed an ample recovery, 

 there ensues, because of the probable opening more or less of the stomata 

 for a second time, for which I have some evidence, a second slackening 

 of growth rates, not very pronounced, to be sure, but evident. It con- 

 sists with this explanation that the second maximum — and this time 

 the greater — is only reached after the evaporation rate has begun to 

 decline; and further that on foggy days the maximum is reached earlier 

 if more gradually and steadily, since on these days the evaporation 

 rates are lower and decline earlier (fig. 5). The data for separate days 

 (table 3) show that on these there was almost invariably a single maxi- 

 mum only, albeit this may fall as early as 13 hour or as late as 17 hour, 

 variation in behaviour which would doubtless correlate with evaporation 

 conditions. It is however of importance to point out that the absolute 

 values for the earlier maximum on sunny days are frequently as high as 

 those of the maximum for foggy days, and similar values may be reached 

 on both sunny and foggy days even at midday, a fact which contra- 

 venes the view that light as such within the limits of availability, acts 

 inhibitorily. 



So far as the data serve to indicate, there seemed to be a tendency 

 for the maximum to be reached later in the afternoon as the season 

 advanced. This tendency may have depended on the decrease of soil 

 mxoisture which without any doubt occurs. It might be argued that a 

 concomitant greater daily shrinkage should also manifest itself. Since 

 this is not evident, however, it may be inferred that during the morning 

 hours the water accumulated in the plant during the night is lost, while 

 the subsequent behaviour, later in the afternoon, involves an immediate 

 draft on the soil moisture. It is most significant that rapid growth 

 rates may be assumed during the period of maximum insolation. 



Relative amount of growth in day and night (table 2; figs. 7, 8). — It will 

 be seen from the above that the maximum day rates of growth are much 

 higher than the maximum night rates. Furthermore, in spite of the 



