DIV. I 



MORPHOLOGY 



37 



chemical and structural differentiation of the cell wall, which thus behaves almost 

 as if it were a living structure. The centrifugal thickening of the walls of cells 

 which have arisen by free cell formation (e.g. ascospores) is effected by the 

 periplasm from which the cells have been cut out (cf. p. 27). Similarly the 

 thickenings of many pollen grains and spores are deposited from without by 

 the protoplasm of the tapetal cells which line the cavities in which they are 

 developed. The protoplasts of the tapetum fuse to a periplasmodium surrounding 

 the young spores or pollen grains ( 31 ). 



In some cases fine striae, running obliquely to the longitudinal axis of the cell, 

 are apparent when the thickening layers are viewed from the surface (Fig. 34). 

 This striation depends either on a distinction in the individual thickeuing layers 

 of regions of different density, the denser frequently projecting into the cell cavity, 

 or (in many Algae such as Cladophora) 



on a wave-like folding of ,the lamellae. 

 If the wall is distinctly stratified the 

 striae in successive thickening layers are 

 usually inclined in opposite directions 

 (Fig. 34). 



FIG. 33. Cell of Fieus elastica contain- 

 ing a cystolith. c. (x '240. After 

 SCHEXCK.) 



FIG. 34. Part of a sclerenchymatous fibre 

 from Vinca major. The striations of 

 the outer layers are more apparent than 

 those of the inner layers. The thick- 

 ness of the wall, as seen in optical 

 section, is also shown, (x 500. After 

 STRASEURGEB.) 



Chemical Nature of the Cell Wall ( 32 ). Although capable of the 

 above processes of growth the cell wall is from the outset not a living 

 portion of the protoplast but a product secreted by the latter. In 

 course of time it can undergo changes of a chemical nature. In 

 living cells it is al \vays permeated by water and swollen, but shrinks 

 correspondingly when the water is more or less completely removed. 

 The lamellae of the wall consist of CARBOHYDRATES, in the main of 

 CELLULOSES, but also of HEMICELLULOSES or PENTOSANES, and as a rule 

 of several of these substances. The cell walls thus never consist of 

 pure cellulose. The celluloses occur in the walls of all plants with 

 the exception of most fungi ; they are polysaccharides, the composition 

 of which is expressed by the formula (C 6 H 10 5 ) n . They stain blue 

 with chlor-zinc-iodide solution but not with iodine alone. This 



