238 Transactions of the Royal Canadian Institute 



in view of the results obtained by Blaauw ('14) but that a reverse effect 

 — that is an inhibitory one — exists cannot be inferred from the evidence. 

 As we have seen, the maximum slowing up of growth occurs during the 

 early morning hours of increasing illumination, but precisely the reverse 

 occurs after the ten o'clock minimum has been attained, and this while 

 the insolation is approaching its maximum values. 



"That light can^ be held to account for the retardation of growth 

 during the morning hours as above indicated has been shown to be an 

 untenable view, since it was found possible experimentally to alter the 

 rates both positively and negatively quite independently of the con- 

 stancy, increase or decrease of illumination, even when this has been 

 increased with respect to the growing part by insolation from three 

 directions. There seems indeed to be no maximum insolation normally 

 occurring in the field which can cause any cessation or inhibition of 

 growth when conditions which insure water supply to the growing part 

 obtain." 



This conclusion accords with not a few critical studies already re- 

 corded. One may cite the results of R. H. Lock ('04) and by A. M. Smith 

 ('06) in whose essays earlier pertinent observations and views are quite 

 fully reviewed. With particular reference to the relation of light and 

 growth, A. M. Smith found "no decisive case of retarding light effect" 

 even under the light conditions of Ceylon, nor did Lock, who worked 

 there two years earlier, also on Dendro calamus. Collins and Kempton 

 ('14) recorded higher growth rates in Zea during daylight than during 

 the night; and Copeland ('16) found for Dioscorea that the "rate of 

 growth of vigorous young shoots is but slightly, if at all, influenced by 

 the illumination" and that "the part of the stem which executes move- 

 ments . . . ceases to elongate in the darkness". In Lagerstroemia he 

 ('13) found however that the tendrils seem to offer a special case in 

 which "the growth region is longer during the night" and that growth 

 then occurs in considerable measure. On this point, however, specific 

 data are not given, since the purpose of the paper was aside from this point. 



On the other hand Lock concluded that there was a marked paral- 

 lelism between humidity and growth^ while temperature had the opposite 

 effect. Although growth was usually greater at night, the opposite 

 could occur in the daytime if temperature and humidity conditions were 

 sufficiently favourable. Lock recorded shrinkage on one occasion in 



1 In my preliminary account ('16, p. 58) this verb was printed by error in the negative 

 but obviously reversed my meaning. The present quotation is taken from the original 

 MS. without alteration. 



2 One may here recall a paragraph from deVries ('74, P- 318) in which emphasis is 

 placed on humidity. 



