7l6 THE AMERICAN NATURALIST. [Vol. 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 (i) 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 a 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 Nageli 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- 

 istry, of the carbohydrates is so complex that great changes of 

 form, bulk, or optical properties may be readily assumed which 

 would quite change the appearance of a structure without, how- 

 ever, necessitating the transportation of new carbohydrate sub- 

 stance to it directly. 



