No. 449-1 STUDIES ON THE PLANT CELL. 391 



lose is so intimate as to resist very severe treatment and there- 

 fore these cell walls are essentially cellulose groups modified 

 chiefly in their physical properties by the presence of foreign 

 substances. The most conspicuous modifications of this charac- 

 ter are lignification, suberization and cutinization. Lignified 

 walls are permeable to water and gases. Several substances 

 have been separated from the cellulose of lignified walls, among 

 them lignone, coniferin, vanillin, etc. Suberized and cutinized 

 walls are largely but probably never wholly impervious to water 

 and gases ; the one is infiltrated with suberin and the other 

 with cutin, substances that resemble one other very closely. 

 Even walls that appear to be pure cellulose have other sub- 

 stance united with them, the most important being pectose and 

 callose. Cell walls frequently become gelatinous or mucilaginous, 

 when the outer layers swell and lose their form or they may be 

 transformed into gums. These changes are well illustrated in 

 the coats of seeds and fruits and among the algae and fungi. 

 The cells of algae frequently secrete gelatinous envelopes or 

 sheaths of substances so closely related to cellulose that were 

 they condensed they would form a firm cell wall. 



The cell wall may grow in two directions by methods quite 

 different from one another. There is first surface growth 

 which results in a stretching of the cellulose membrane (growth 

 by intussusception) . And second there may be growth in thick- 

 ness by the formation of successive layers of cellulose inside of 

 one another, giving the wall a striated structure (growth by 

 apposition). The second type of growth is chiefly interesting 

 since it makes possible many peculiarities of structure, because 

 the newly formed layers may not be deposited uniformly inside 

 the primary wall. In some cells the secondary thickenings 

 have the form of rings or spirals or a reticulate structure. The 

 reticulate condition passes insensibly into the pitted cell in which 

 the secondary layers cover the greater part of the surface leav- 

 ing the primary wall only exposed at the pits. Further dis- 

 cussion of these cells falls more within the range of histology 

 than the purposes of this paper. 



The cell wall offers a very interesting field of research among 

 the thallophytes and especially in the lower groups where we 



