142 
Sydney II. Vines. 
rection. The question at once suggests itself, does light directly infiuence 
the growing cells, and if so, whal is the nature of its action 1 ? 
An altempt lo indicate such an iufluence was made by Batalin 1 ). 
He suggesled, that ihe usual smallness of etiolated leaves of Dicotyledons 
tnighl be due to an arrest of the process of cell-division, a process which 
he regarded as being favoured by the presence of light. Sachs 2 ) has 
however pointed out, that cell-division is only a consequence of growth. 
The leaves remain small, not because their cells do not divide, but be- 
cause their cells do not grow. A good example of this is afforded by 
the smaller Algae. Their cells divide only during the night, most pro- 
bably because il is then that growth is aclively proceeding. 
In order lo obtain a clear idea of the problem the physical condilions 
of growth must be briefly considered. These conditions are principally 
three : 1} the growing cell must be supplied vvith a sufficienl quanlity of 
nutrilious material; 2) the temperature of the surrounding medium must 
not vary beyond certaiu limits; and 3) the cell must be supplied with 
water so that it may be tense, or as it is technically termed, in a state 
of turgidity. Of these three conditions the first two are but litlle affec- 
ted by light. In darkness as well as in light the mycelium of Phycomyccs 
eould always obtain an adequate supply of food, and the rise of tem- 
perature which generally took place when the plant was exposed to light, 
was never sufficieully great to endanger the life of the plant. Under 
these circumstances, the action of light must have affected the last of the 
three conditions: — it must, in some way or olher, have modified the 
turgidity of the growing cell. 
The imporlance of a turgid condition for the process of growth in a 
cell was first insisted upon by Sachs 3 ]. He defines il as follows. — »By 
turgidity we understand the hydroslatic pressure which the water absorbed 
by endosmosis exerls upon oll parts of the cell-wall equally, and which 
reacts on the cell-contents in consequence of the elasticity of the cell- 
wall. Tlius, in a turgid cell, the cell-wall is lense and the cell-con¬ 
tents are under pressure«. The result of this state of turgidity is lo 
cause the cell-wall to Stretch as much as possible. ln order that it may 
be able tlius lo yield to the pressure, from within it must be salurated 
with w : aler. The process of its extension consisls in the gradual Separa¬ 
tion frome one another of the minute solid particles of which il is com- 
posed (»mreellae« Nageli und Schwendener Mikroskop— »tagmata«. 
Pfeffer Osmot. Untersuch.), the spaces between them being occupied by 
water. If the cell be young and eapable of growth, new micellae are 
1j Bot. Zeitg. 1871. 
2) Lehrbuch 4. Auflage p. 733. 
3) Lehrbuch 4. Auflage pp. 741—Soä. 
