VI. The lnfluence of Light upon tlie Growlh of unieellutar Organs. 
143 
dcposiled in the walery areas, and Ihe form assumed by the cell-wall in 
its extended condition is rendered permanent. 
The turgidity of a cell may therefore be modified in lwo ways. In 
the first place, the extensibilily of the cell-wall may either increase or 
diminish, and the reaclion which it exerts upon the cell-contenls will 
van accordingly. ln the second place, the osmotic properties of the celI— 
contents may be so allered, that the amount of fluid contaiued within the 
cell may Vary, and therefore the pressure exerted upon the cell-wall front 
within will vary also. 
A diminution of the extensibilily of a cell-wall occurs as the cell be- 
comes older, and as the cell-w all itself undergoes diflerenliation 1 ). The 
observations of Kraus and Kocu tend to shew, that the aclion of light is 
favourable to the thickening and diflerenliation of cell-walls. Even if it 
be assumed that the retardalion of the growlh of a liypha of Phycomyces 
depends upon a diminished extensibility of its cell-wall, the facl that it 
is so rapidly produced makes it difficult to believe that a thickening of 
the cell-wall is the immediate cause. It seems more reasonable to ima- 
gin.e that light exerts its influence directly upon the rnicelhe in such a 
way as diminish their mobility. Pfeffer 2 : has already suggesled the pos- 
sibility of such a direct action of light upon cell-walls, but any positive 
evidence upon the subject is not to be oblained. This being the case, 
it will be well to proceed to the discussion of the secoud of the two 
possible ways in which the turgidity of a cell may be modified, in Order 
to obtain a more satisfactory explanation. 
The wall of a growing cell is lined by a layer of proloplasm, which, 
so long as the cell is turgid, is in close contacl wilh it. It is evident, 
therefore, that the endosmosis, by means of w r hich the cell is maintained 
in a turgid condition, has to take place through this protoplasmic layer. 
U may be, that the eHVct of light in rctarding the growlh of a cell is pro¬ 
duced by some change in the physical properties of the protoplasm in 
consequence of which turgidity is diminished. A short discussion of the 
use of the proloplasm in producing and maintaining the turgidity of a 
cell will aflord some grounds on which to form an estimale of the pro- 
bability of this Suggestion. 
Näoeli 3 ) was the first to point out, that the protoplasmic layer lining 
the cell-wall ofl'ers a considerable resistance to the passage through it of 
certain substances, more especially colouring mallers, which are dissol- 
ved in the ccll-sap, although the cell-wall easily nllowed them to pass. 
De Vries 4 ) repeated Nagku’s observations upon the parencbymatous cel Is 
1) Nägeli: Pflanzen-phys. Untersuch. lieft. I. p. 24. 
2) Periodische Bewegungen p. 119. 
3 Pflanzen-phystol. Untersuch. Heft I. 
4) Sur la pertnCabilite du protoplosma des belteraves rouges. Arch. Neerland. 1871. 
