LIMITING FACTORS TO GROWTH MEASUREMENTS. 349 



It may be worth while however to note shortly their bearing 

 on the observations and theory of the present paper. 



The interest of the record for us is that our theory indicates 

 that for species of Agave cases might be found when ■ the 

 growth would be so great that the necessary water supply 

 would be lacking and would become the limiting factor. This 

 would, be expected at the highest temperatures. No such 

 case has come under my own direct observation. De Vries' 

 measurements of Agave, while showing at first some relation 

 to temperature, afterwards showed a decided relation to the 

 humidity as shown by a cessation of growth on fine mornings 

 while some growth was recorded on dull or rainy mornings. 



It is probable that this occurred at a lower temperature 

 than those recorded in Ceylon, since the measurements were 

 made in Holland (though as the time was summer, it is not 

 certain) and this lack of water supply did probably not occur at. 

 a time of very high temperature. It is noted however that the 

 experimental plant had no roots and the absorption of water 

 took place by the porous dead mass covering the living root- 

 8tock. It is quite probable that under such circumstances 

 the water supply might be limiting at a much lower tempera- 

 ture than occurred in Ceylon. Again it is quite possible that 

 the absorptive power of the roots may have been checked by 

 the comparatively low temperature of the soil which may 

 have been considerably lower than in the cases measured by 

 me in Ceylon. All this however is conjectural in the absence 

 of figures. There is some difficulty in giving any explanation 

 for the whole 24 hours in this case, for it is recorded that while 

 growth ceased or almost ceased during the period 9 a.m. to 

 12 noon, yet the period 12 noon to 3 p.m., although probably 

 hotter and with lower humidity, yet always showed consider- 

 able growth. Sachs explains the cessation of growth in the 

 morning as due partly to the retarding effect of light, but in 

 view of my own results this scarcely seems possible. 



A result which would be easily explained on our theory ie 

 that before August 10 when the shoot was in its earlier stages 



