VI. The Influenee of Light upon ihe Growlh of unicellular Organs. 141 
only occurs when the plant is exposed lo light. The observatious of 
Weber 1 ) upon the absorption of phosphoric acid and lime, of Schulze 2 ) 
upon the formalion of sulphuric acid, and of Rauwenhoff 3 ) upon the 
occurrence of crystals of calcium oxalate and of tannic acid in etio- 
laled and in green planls, all point to the aclivity of light in the 
Chemical processes of the plant. The facl that in verj man} cases the 
leaves or Dicotyledonous plants grown in the dark remain small, has been 
explained by Kraus 4 ) on the theory that leaves can only grow when they 
can assimilate 5 ). This explanalion has been shewn lo be incorrccl by 
Batalin 15 ), who found, that if eliolated leaves be exposed lo diffuse day- 
lighl lor a period which is not sufficiently long to cause the formation 
ol Chlorophyll, they will grow. It is well-known that ihe cotylcdons of 
many plants do not grow in darkness, although their cells are filled with 
slarch or oil, and this is also the case with the leaves of green plants 
which have been removed into the dark. Kraus 7 ) attributes this arrest 
of growth to the cessation of the process of conversion of slarch inlo Cel¬ 
lulose. It is difficull to reconcile this explanalion with Ihe fact, that Ihe 
leaves of many Monocotyledons aud those of inany Dicolyledons s ), when 
grown in the dark, attain as great as a size, or even a greater, than that 
which they would have normall y possessed. Moreover rools grow in 
complete darkness, and etiolated stems become excessively elongated. In 
all these cases the process of ihe conversion of slarch inlo cellulose must 
be in full aclivity'. It is therefore not clear why it should be assumed 
that so fundamental a process as Ihat of the conversion of starch inlo 
cellulose is arrested in certain cases only , by the absence of light. It 
cannol be doubtcd, however, that, in darkness, certain Chemical proces¬ 
ses, which are essential, as regards the leaves, lo the growth of the cells, 
and as regards both leaves and stems, to the differentiation of the lis- 
sues, are suppressed. These are only some of the effeels produced by 
eüolation. There are many others, of which the non-formation of Chloro¬ 
phyll is the most slriking. 
If this evidence be considered sufficient to prove the correctness of 
the Statement, that the Chemical processes which are essential to the nu- 
trition of plants are favoured by the action of light, the explanalion of 
the retarding action of light upon growth must be soughl in another di- 
t) Landvv. Versuchsst. XVIII. 
2) Landw. Versuctisst. XIX. 
3) Over de Oorzaken der abnormal.! Vormen etc. Acad. Amst. XI. 1877. 
4 Jahrb. f. wiss. Bot. Bd. VII. 
5) This view is discussed in another paper. Arb. Heft V. 
6) Bot. Zeitg. 1871. 
7) loc. cit. p. 214. 
8) Sachs: Einfluss des Tageslichts eic. Bot. Zeit. 1863. Beilage. 
