6 ELEMENTS OF PLANT ANATOMY. 
tubes by the incoming water, which fills the channels before 
occupied by air without affecting the size of the wood. 
It is not known with certainty to what extent the process 
of evaporation can be carried. It is supposed, however, that 
under certain conditions organized structures may lose all the 
water which ordinarily forms the films about the micellae, so 
that the latter may he in actual contact; and that when water 
is again placed in reach of the structure, its molecules have to 
make an opening for themselves by forcing the micellae apart. 
Nothing is definitely known concerning the shape of these 
micellae. There are several reasons for supposing them to be 
prismatic rather than spherical. That they lie in contact after 
extreme evaporation is indicated by the non-appearance of air, 
which if present would cause a difference in the refraction of 
light. This contact of surfaces could not take place were the 
micellae spherical. Their behavior under the action of polar- 
ized light indicates crystalline surfaces and lines of cleavage. 
Finally the shrinking and swelling of wood in different pro- 
portions, longitudinally, radially, and tangentially, indicate a 
difference in the three axes of dimension in the micellae. 
In order to apply this theory concerning organized structure, 
the cell must be studied in its various stages of growth begin- 
ning with its first formation near the growing tip of the plant. 
Here the cavity of the cell is seen to be filled with protoplasm 
which appears to be nearly, if not quite, homogeneous. As 
the cell becomes older, one or more vacuoles make their 
appearance. These vacuoles are simply water or sap-filled 
spaces which arise from the cell taking in more water than 
can be held in the interstices between the micellae of the 
protoplasm. 
The theory respecting the physical nature of protoplasm is, 
that in its active state its units of structure are surrounded by 
films of water, which may be increased to a certain thickness ; 
when this limit is reached no more water can be held, and if 
