76 



BOTANY 



PAP.T I 



of tissue (p. 127). Cells on the surface of plants have usually only 

 their outer walls thickened (Fig 81 e). 



Unequal thickening of the cell wall is most noticeable when it is limited to 

 definite small areas. This is the case with those lens-shaped thickenings of the 

 outer walls of the epidermal cells of the upper surface of foliage leaves, which 

 are of frequent occurrence and, according to G. Haberlandt ('^), act as convex 

 lenses to concentrate the light (Fig. 82). By the thickening of cell walls at 

 special points, protuberances projecting into the cell cavity are formed ; in this 

 way the formations known as cystoliths arise. Certain large cells in the leaves of 

 the India-rubber plant {Ficus elastica) contain peculiar clustered bodies, formed 

 by the thickening of the cell wall at a single point (Fig. 83). In their 

 formation a stem-like body or stalk first protrudes from the cell wall ; by the 



addition of freshly deposited layers 

 this becomes club shaped, and, by 

 continued irregular deposits, it finally 

 attains its warty surface. 



So far only centripetal wall 

 thickenings have been described. 

 Cells, the Avails of which are 

 centrifugally thickened, can 

 naturally only occur where the 

 cell walls have free surfaces. The 

 outer walls of hairs generally 



■V^^^^s^ 



Fir..s4-J, Pollen-grain of ft(CHr6itaPej)o in surface show Small inequalities and pro- 



view, and partly in optical section, rendered jectioUS. The SUrface Walls of 



transparent by treating with oil of lemons sporeS and polleu grains ^Fig. 84) 



(x 24U) ; JJ, part of transverse section of pollen ^ i o v & / 



grain of Cucurbita verrucosa (x 540). 



show a great variety of such 

 centrifugally developed protu- 

 berances, in the form of spines, ridges, reticulations, and bands of 

 characteristic structure. 



Cell-Wall Substance ("-). — The most important constitutent of 

 cell walls is cellulose. It is present in the cell walls of all plants, 

 except in those of the majority of Fungi. 



Cellulose is a carbohydrate, one of the polysaccharides of which the chemical 

 composition is expressed by the general formula (C6H]oOg)n. It is insoluble in 

 dilute acids or alkalies ; even concentrated pota.sh solution does not dissolve it. By 

 the action of concentrated sulphuric acid it is converted into dextrose and becomes 

 soluble, and it is dissolved by ammonia oxide of copper. After treatment with 

 sulphuric or phosphoric acid, iodine will colour it blue ; it shows a similar reaction 

 when exposed to the simultaneous action of a concentrated solution of certain salts 

 such as zinc chloride or aluminium chloride, and of iodine. Accordingly, chlor- 

 zinc-iodide, on account of the blue colour imjjarted by it, is one of the most con- 

 venient tests for cfdlulose. Gilson (^•') ol)tained cellulose in a crystalline condition in 

 the form of sphaerites or dencb-ites. The name of hemicellulose is given to a series 

 of substances which are nearly related to cellulose but arc hydrolysed by even 

 dilute acids and yield sugars other than dextrose. 



