V. The Influence of Light upon the Growlh of Leaves. 
127 
It now remains to discuss Stebler’s views as to the cause of the 
periodicitv observable in tbe growlh of leaves. He says 1 ), that the linear 
leaves of Mouocotyledons witli vvhich he experimented exhibited such a 
daily periodicily, that the rapidily of growlh always rose with increasing, 
and feil with diminishing intensity of light. The maximum of growlh 
corresponded with the greatest intensity of light, and the minimum oc- 
curred shortly before daybreak. Ile explains these facts by regarding the 
process of assimilation as their cause, or as he States it, growlh rises 
with increasing assimilation, and falls when assimilation diminishes. With 
reference to the growlh of the leaves of Dicotyledons he says that the 
daily period is modified in such a way that, afler the occurrence of the 
maximum of growlh in the forenoon, a retardation takes place, so that 
the curve of growth gradually sinks, unlil the minimum is reached shortly 
before daybreak on the following moruing. After dawn growlh rapidly 
inereases and attains its maximum in the forenoon. lf the light be very 
intense, the maximum occurs earlier, if less intense, it occurs later. Ilis 
explariation is that in the case of the leaves of Dicotyledons, the daily 
period is produced by assimilation as in the case of the leaves of Mono- 
cotyledons. The retardation which occurs in the course of the day afler 
the maximum of growth has been attained, he ascribes to the influence 
of light. 
It is evident that Kraus’ theory of the »self-nulrition« of leaves has 
afforded the stand-poinl front which Stebler regards his observations. 
It has already been shewn that this theory is in conlradiction to expe¬ 
rimental evidence, and therefore any inferences which roay have been 
drawn from it cease to be tenable. Still the question at issue, namelv, 
whether or not the influences which produce the daily periodicily in the 
growth of leaves are different from those which produce a similar pe- 
riodicitv in the growth of olher vegetable Organs, is of such importance, 
that a closer examinalion of Stebi.er’s inferences is desirable. 
ln the first place, a careful analysis of his figures shews, that the 
coincidence of the maximum and minimum of growth with the maximum 
and minimum of assimilation is by no means so frequent an occurrence 
as might be supposed. The following tables, w hich include all his expe- 
riments upon plants under normal condilions, bring this out very clearly. 
In drawing up these tables, the observations made upon one plant 
during one day are regarded as forming one experiment. The period 
of the greatest intensity of light is, in some cases, between 10 a. m. 
and 2 p. m., in others, between 9 a. m. and 3 p. m. This difference 
is due to the fact that in some of Stebler’s observations the measure- 
ments were made everj four hours, in others, every three hours. Si¬ 
ll loc. cit. p. tSi. 
