THE PLANT-CELL 



51 



Fungi and Lichens by a substance Fungus-cellulose somewhat 

 different in composition from ordinary cellulose, and there are some 

 other modifications of it. Some of these which are more readily 

 attacked by acids and alkalis have been termed " Hemicelluloses." 

 In Fungi, moreover, the cellulose may contain chitin, in this respect 

 resembling some animal tissues, and the Bacteria and other Schizo- 

 phy tes seldom show an 

 unmodified cellulose 

 wall. How far these 

 changes are secondary 

 is not certain. Most 

 cells, as they grow 

 older, show modifica- 

 tions of the wall, which 

 may be of two kinds, 

 mechanical thick- 

 *ening and chemical 

 changes. 



Thickening of the Cell- 

 wall. The older cell- 

 wall often shows a 

 marked stratification, 

 which is sometimes 

 very pronounced; e.g. 

 old cells of Cladoph- 

 ora. Here the thick- 

 ening is apparently due FlG - 33.Fimbriaria C'alifornica. A, B, elaters. C, 

 fn fV.P r>QiHn' nf -D, sections of spores, showing unequal thickening of 



the cell-wall (A, (J, X 600; B, D, X 300). 

 new layers of cellulose 



on the inner surface of the wall. Where the thickenings are upon 

 the outside of the cell-wall, as in the sculpturings of certain 

 Desmids, or the formation of hairs upon such Algae as Coleochsete, 

 it is probable that the cell-wall is more or less completely per- 

 meated by the living protoplasm, which, in the case of the Desmids 

 at least, has been shown to pass through the cell-wall by means 

 of extremely fine pores. The sculpturings upon the outer surface 

 of the spores of Mosses and Ferns and the corresponding pollen- 

 spores of the Phanerogams can in most cases be attributed to the 

 activity of the protoplasm surrounding the developing spores. 

 Where the deposit on the inner surface of the wall is unequal, 

 there result markings of various kinds, such as the characteristic 

 spirals, pits, and reticulations found upon the walls of the tracheary 

 or water-conducting tissue of the higher plants. 



Much controversy has been aroused in regard to whether the wall 

 grows by simple apposition of new material, or whether it may grow 



