194 INTRODUCTION TO CYTOLOGY 



particularly Mangin (1888-1893) it has been found that the chief constitu- 

 ents of the newly formed cell walls of plants are pectose and cellulose — 

 that the primary wall or middle lamella consists of pectose, the secondary 

 layer of pectose and cellulose, and the tertiary layer of cellulose. These 

 substances however, rarely exist in the wall in pure and unmodified form. 

 The pectose of the primary layer changes later to insoluble pectates, 

 especially the pectate of calcium, while the secondary and tertiary layers 

 very soon become greatly changed in composition, not alone through the 

 addition of a variety of new substances, but also through an actual trans- 

 formation which in some cases appears to be complete. For example, the 

 secondary and tertiary layers of xylem cells, although at first containing 

 much cellulose, may later become so completely transformed into or re- 

 placed by lignin that they show no reaction whatever to cellulose stains. 

 In some cases the primary wall may undergo a certain amount of lignifi- 

 cation also. The walls of many cells become heavily impregnated with 

 cutin or suberin, the latter substance being responsible for the peculiar 

 character of corky tissues. Infiltration by cutin, or "cutinization," is 

 to be distinguished from "cuticularization," by which is meant the secre- 

 tion of a layer of cutin (cuticle) on the outside of the cell. A variety of 

 mineral substances, such as silica, calcium oxalate, and calcium carbonate, 

 as well as more complex organic compounds, such as tannin, oils, and 

 resins, are often deposited in the walls of old cells. The heartwoods 

 of trees owe their qualities largely to the presence of these additional 

 materials. 



In spite of these modifications, however, it is still true that cellulose 

 is the substance chiefly characteristic of plant cell walls in general. Al- 

 though cellulose has been identified in certain animals, the membranes of 

 practically all animal cells are composed of other substances, such as 

 keratin, elastin, gelatin, and chitin. In the fungi also the role of cellulose 

 appears to be played in part by chitin. 



The Walls of Spores. — Special attention has been given to the develop- 

 ment of the elaborate walls, or coats, of the spores of various plants in a 

 number of investigations. Strasburger (1882, 1889, 1898, 1907) con- 

 cluded that such coats arise by two general methods: (1) by the growth in 

 thickness (by apposition) of the original wall of the spore cell through the 

 activity of the protoplast, as in the pollen grains of Malva and other angio- 

 sperms, and (2) by a deposition of material upon the original wall by the 

 tapetal fluid in which the young spores lie, as in the case of the megaspore 

 of Marsilia. 



The highly specialized coats of the megaspore of Selaginella have been 

 most intensively studied, particularly by Fitting (1900, 1906) and Miss 

 Lyon (1905), whose accounts disagree in several points. At the close of 

 the tetrad division there is formed about each young spore a thick gela- 

 tinous ''special wall," at the inner surface of which, according to Fitting, 



