390 THE AMERICAN NATURALIST. [Vol. XXXVIII. 



3. The Cell Wall. 



Many of the chief peculiarities of plant organization and 

 activities are due to the presence of the cell wall, its influence 

 on structure and mode of life. The cell wall is not an excretion 

 from the cell like a mineral shell but is formed by the direct 

 change of portions of the protoplasm. The. regions concerned 

 may be the outer plasma membrane, the vacuolar plasma mem- 

 brane or the substance that makes up the spindle fibers which 

 form the cell plate. These structures are all kinoplasmic in 

 character and have to do with the formation of cell walls in 

 various ways which will be described in Section II under the 

 topic "The Segmentation of the protoplasm." The transforma- 

 tion of finely granular films of kinoplasm into cellulose is not 

 well understood but there is an evident solution of the granules 

 (microsomata) and the change of the resultant substance into 

 the cell wall. As a chemical process this change means the 

 replacement of molecules of an albuminous nature by those of a 

 carbohydrate substance. The most complete account of the 

 cell wall is that of Strasburger, '98. 



Cell walls are chiefly composed of cellulose, but other sub- 

 stances are always present, modifying the structure in various 

 ways to give widely different properties. These modifications 

 are generally due to infiltrations of foreign substances but some- 

 times cell walls become incrusted with mineral deposits. The 

 group of cellulose compounds is very large and it is extremely 

 difficult to identify the various substances in structures so small 

 as the cell walls. For a detailed treatment of the chemistry of 

 the cellulose group the reader is referred to Cross and Bevans, 

 '95, and for a general account to Pfeffer, : 00, p. 480-485 . There 

 are microchemical tests for cellulose that give good reactions 

 for most tissues but which cannot be relied upon for some walls 

 (as in fungi and many algae) yet it is well understood that the 

 cell walls of these organisms are from the biological point of 

 view essentially the same as for other plants. The cell walls of 

 some fungi are very largely composed of chitin. 



Several substances known to be present in cell walls give 

 them marked characteristics. Their association with the cellu- 



