8 ELEMENTS OF PLANT ANATOMY. 
thin membrane, which differs from it in physical structure. 
This membrane exists also on the outer part of the fine proto- 
plasmic threads, and plays an important part in the processes 
of plant nutrition. While the discussion of these processes 
belongs properly to physiology rather than to anatomy, the two 
subjects are here so closely connected that it is impossible to 
consider one without including the other. 
The plant cell in its ordinary active condition consists of, 
at least, these four parts: wall, protoplasm, nucleus, and vacuole. 
According to the explanation given of the organized structure 
of the wall, it is readily seen that only substances in solution 
can pass in and out of the cell without injuring it. The sub- 
stance dissolved may be either organic or inorganic. Very 
little is definitely known concerning the actual method by 
which this exchange of material is effected. Experiments 
show, however, that the protoplasmic membrane is the deciding 
factor as to what substances may find their way in and out of 
the cell. | 
Various substances are found dissolved in the water of the 
vacuoles. These may be inorganic salts which have been taken 
into the cell by osmosis, or organic substances formed there by 
the metabolism of the living cell, or taken in from without as 
the product of some other organism. 
Now if such a cell be placed in contact with pure water, or 
with a solution whose concentration is less than that of the 
liquid in the vacuole, the laws of diffusion are such as to cause 
a flow of water into the cell. In consequence of this incoming 
water the cell becomes increased in size and the wall stretched 
and distended. When sufficient water has been taken in to 
cause the pressure on the stretched wall to be greater than that 
of the atmosphere without, this condition is known as turgor,} 
and the cell is said to be turgescent. 
4 
1 For a more extended description of turgor, see Pfeffer, “ Zur Kenntniss--. 
der Plasmahaut und der Vacuolen,’’ page 297, The author in speaking of the 
