716 THE AMERICAN NATURALIST. . [Vor. XXXIX. 
reasons for regarding such transformations as an actual increase 
in the carbohydrate material for it is clear that the substance is 
a body with a greater amount of water in its organization than 
is present in the more usual forms of cellulose compounds. 
But there are some cases which are not so easily understood and 
perhaps the most widely known are the megaspore walls of cer- 
tain species of Selaginella. These spores are remarkable for a 
differentiation of the spore wall in which the outer layer seems 
to be entirely separated from the inner by a space and yet is 
able to increase enormously in size and take on marked pecul- 
iarities of structure, but apparently without any relation to the 
protoplast. It may, however, be justly questioned whether the 
apparent space between the inner and outer wall is really a 
cavity and may not be filled with plastic material which holds 
the two walls in intimate organic relation to one another and to 
the protoplast. Miss Lyon has recently given this subject at- 
tention and announced her belief that the latter condition ob- 
tains. Her conclusions will be awaited with interest. 
As regards the way in which a cell wall increases in size we 
are still limited to the two conceptions termed (1) growth by 
apposition and (2) growth by intussusception. The first method 
consists in the laying down of successive layers by the plasma 
membrane and results in a thickening of the cell wall. It is of 
course à comparatively simple process. Growth by intussuscep- 
tion is a stretching or expansion of the substance which seems 
to be greatly increased in quantity although the morphology of 
the structure remains the same. The current explanation out- 
lined by Nägeli assumes that new molecules of carbohydrates 
are intercalated among the old. It seems more probable that 
the increase in bulk is due to some modification or rearrange- 
ment of existing molecules, involved, perhaps with an increase 
of material but not through the actual intercalation of new 
molecules of the same or original carbohydrates. The chem- 
iia . i is so complex that great changes of 
mie Aa no properties may be readily assumed which 
ge the appearance of a structure without, how- 
ever, necessitating the transportation of new carbohydrate sub- 
stance to it directly. 
