Formed Constituents of Organisms 153 



Cellulose is a substance that plays an important part in plant 

 tissues alike phylogenetically and structurally. The pure cel- 

 lulose of most primary cell walls is largely retained in tissues 

 "throughout the plant series. But, TN^ith the transition from 

 a primitive aquatic to a subaquatic and eventually to a terres- 

 trial mode of life, three important modifications of it have been 

 laid down by the protoplasm, viz., the pectin, the lignin, and the 

 cutin types. The two former will meanwhile engage our attention. 



Pectin seems to occur as an ordinary wall substance in even 

 the lower algse, but it and lignin become set apart as the strength- 

 ening materials for cell walls of terrestrial plants. Deposited 

 as successive fine layers from the protoplasm, and on the in- 

 terior of the pure cellulose membrane, they give to the latter, 

 when they are of a pectin nature, great tenacity and flexibility 

 as in bast fibers, e. g., of flax; or, when of a lignin nature as 

 in wood fiber, they give at once strength, hardness, and con- 

 siderable tenacity. 



Now from the group of the liverworts or Hepaticse, where 

 they first appear conspicuously as secondary deposits, upward 

 to the highest plants these appear and disappear wholly accord- 

 ing to environal relations. Thus in climbing terrestrial aroids 

 the cell walls in roots, stems, and petioles may often be heavily 

 thickened by pectin or lignin deposits; in soft and short-lived 

 shade-loving species it may be small in amount; while in such 

 aquatics as Orontium only feeble deposits are locally formed. 

 Comparison of Polygonum ampkihimn in its aquatic variety 

 mth others like P. lapathifoliian that are upright resisting 

 types, or of Ranunculus aquaiilis in its stem tissues with such 

 an upright species as R. bulbosus, reveals like striking differ- 

 ences. But scores of such examples might be cited from allied 

 species or genera of a family. From the algae upward for 

 pectin, from the moss alliance upward for ligin, then, these 

 definite allies of cellulose may appear and disappear from 

 tissues according to environal conditions. 



Cutin. The nearly related and highly important cutin is 

 of prime value as a protection of surface tissue cells from over 

 evaporation. So while doubtfully present in some of the 

 Schizophyceae this substance becomes a wall modification 

 that appears and disa})pears in amount and even in recognition, 

 in practically every group from the Hepaticae upwards. Here 

 again also one notes that in aquatic species or genera of a 

 family it may scarcely if at all be recognizable by chemical 

 test, while in xerophytic plants of perennial growth, like the 

 leaflets of cycads, or the tissues of yuccas, of aloes, of agaves, 

 of eucalypts and many others, it may attain great thickness. 



