No. 466.] STUDIES ON PLANT CELL.— VIII. 715 
And similarly the cross wall which takes the position of the cell 
plate at the end of mitosis is not developed from the transfor- 
mation of a film of protoplasm but is laid down between two 
surfaces that separate to form a thin vacuole which later spreads 
to the edge of the cell and the wall is deposited between these two 
membranes which are almost in contact. There are a number 
of cases in which large strands or masses of protoplasm have 
been described as changing directly into cellulose but it is prob- : 
able that these examples upon further study will exhibit the 
same relation of the cellulose substances to plasma membranes 
as in the typical cases of wall formation. There are many inter- 
esting examples of cellulose formation whose precise relation to 
the protoplasm has not yet been determined. 
Respecting the exact method by which a cellulose wall is laid 
down by a plasma membrane there is very little real informa- 
tion. It is clear now that the cellulose is not a secretion from 
the plasma membrane comparable to a mineral shell. There is 
much evidence that protoplasm is actually sacrificed in the de- 
velopment of cellulose. There are numerous illustrations, as in 
the tracheids and other cells empty of protoplasm, where the 
final secondary thickenings are deposited as the protoplast grows 
smaller and eventually disappears, a large part of its substance 
evidently contributing to the deposits which are members of the 
cellulose group. But of course it cannot be supposed that the 
molecules of the proteids are changed directly into those of the 
carbohydrates. Nevertheless it does seem clear that the carbo- 
hydrates appear simultaneously with the disappearance of the 
proteids and occupy the position formerly held by the latter. It 
is probable that with the splitting up of the proteid molecule, 
carbohydrate material is formed which displaces the proteid sub- 
stances. So in a broad sense the cellulose deposit actually does 
represent a transformation of a plasma membrane. 
The evidence in general favors the view that the wall, lamellze, 
and other deposits of cellulose only increase in amount when in 
actual contact with a plasma membrane. Some apparent excep- 
tions to this principle are easily understood. Thus cell walls or 
portions of such may swell greatly and become much softer in 
consistency and perhaps even mucilaginous. There are no 
