130 
Sydney II. Vines. 
aclion of light, whcreas (he daiiy periodicitj observed in the leaves of 
the latter group of plants is exclusively a function of assimilalion. 
It is difficull to accept this explanation, for il is nol evident why 
the leaves of Dicotyledonous plants should be held to be more sensitive 
to the relarding aclion of light than those of Monocolyledons. The ob- 
servations of Sachs *) upon the development of etiolaled leaves led him 
to conclude that those leaves whicli, under normal condilions, altain a 
considerable lenglh in more or less complete darkness, in consequenee 
of being invested by older leaves, beeorne excessively elongated when 
etiolaled. This is the case with the leaves of most Monocolyledons, and 
it cannot be denied, therefore, that the relarding action of light has an 
inlluence upon their growlh. 
Ilaving eslablished this fact, it is natural to infer, that the periodicitj 
observed in the growth of the leaves of these plants is, to a great ex- 
tent, produced by the retarding action of light. A comparison of the 
resulls obtained by Sachs , Prantl and Stehler will afford a rneans of 
tesling the value of this Suggestion. 
It has alreadj beeu mentioned that ihe |»eriodicity of growth of in- 
ternodes is attributed by Sachs to the relarding action of light, and that 
the occurrence of the maximum during the day and of the minimum in 
the evening or during the night, is due to the fact that this action of 
light does not make itself instantaneously manifest, and that it persisls 
after the exposure to light has ceased. This view is supporled by the 
observalions of Prantl upon the growth of the leaves of Dicotyledons. 
lf the curves II, III, IV, V, VI given bj Stehler be compared with those 
given by Sachs and Prantl, they will be found to differ principally (herein, 
that the highest points of Stebler’s curves occur somewhat later in the 
day, and the lowesl points l'arther on in the night, than the correspon- 
ding points in those of Sacus and Prantl. This ditference, however, of- 
fers no obslacle to the Suggestion that they are susceplible of a common 
explanation. In fact Stebler’s curves can be intelligbly explained upon 
the some principle as those of Sachs and Prantl. The above mentioned 
peculiarily of Stebler’s curves is probably to be traced to a more gradual 
aclion of light upon the growing cells of the leaves of Monocotyledonous 
plants. This greater slowness of action is to be attributed, to some ex- 
tenl at any rate, to the small amounl of light which can penelrate to 
the growing cells of these leaves. Stehler endeavours to shew that some 
light reaches these cells under ordinary circumstances, and that this light 
contains some of the highly refrangible rays, but he places more reliance 
upon his experimenls with plants in which the growing leaves had been 
fully exposed to the light. The curve V, for example, rej>resents the 
1) Bot. Zeit. 1863 Beilage p. H. 
