MORPHOLOGY 81 



caustic potash. In other respects, the reactions given by cutinised cell walls with 

 chloroiodide of zinc or solutions of caustic potash are almost identical with those 

 of suberised cell walls. 



"While after lignification cell walls are still permeable to both 

 water and gases, suberisation or cutinisation renders them impervious. 

 Accordingly, suberised and cutinised cell walls are found especially in 

 the surface of plants, as a means of protection and preservation. 



The layers of the cell walls of some cells, particularly the super- 

 ficial cells of certain fruits, as of Sage, and of numerous seeds, such as 

 Flax and Quince seeds, become mucilaginous, and swell in water to a 

 slime or vegetable mucus, which, according to G. Klebs, serves the 

 purpose of attaching the seeds to the soil. The internal cells of some 

 leguminous seeds with a mucilaginous endosperm, such as the seeds of 

 the Carob tree (Ceratonia Siliqua), have similar mucilaginous layers, 

 which serve as reserve substance. Firm cell walls can also be trans- 

 formed into GUM, as is so often apparent in Cherry and Acacia trees, 

 portions of whose woody cells often succumb to gummosis. 



The several varieties of gums and vegetable mucus react differently, accord- 

 ing as they are derived from cellulose, callose, pectose, or from allied substances. 

 According to Mangin they may be microchemically distinguished by their 

 reaction with ruthenium red, which stains only such as are derived from pectose 

 or related substances, such as the mucilage of the seeds of the Cruciferae and 

 Quince (Oydonia), the mucus cells of the Malveae, the gums of the Cherry and 

 Acacia, the gum tragacanth from Astragalus gummifer. The mucus of Orchid 

 tubers, on the other hand,- is related to cellulose, and remains uncoloured with the 

 same reagent. 



The cell walls of the seeds of many Palms, as also those of Omithogalum 

 (Fig. 84), have strongly developed thickening layers, which are full of pits. These 

 thickening layers are lustrous white, and, as in the case of the seeds of the Palm, 

 Phytelephas macrocarpa, may attain such a degree of hardness as to be technically 

 valuable as vegetable ivory. Such thickening layers may contain other carbohyd- 

 rates in addition to cellulose; thus the cell walls of the seeds of Tropaeolum and 

 Paeonia, contain an amyloid, which turns blue even with iodine alone. These 

 thickening layers are dissolved during germination, and are accordingly to be 

 considered as a reserve substance of the seeds. 



Cell walls often become coloured by tannin or derivative sub- 

 stances ; in this way, for instance, the dark colour is produced which 

 is often seen in the shells of seeds and in old wood. The colours of 

 the woods of economic value are due to such discoloured cell walls. 

 Inorganic substances are often deposited in large quantities in old cell 

 walls. Among such substances calcium oxalate is often met with, 

 commonly in crystal form ; also calcium carbonate, although perhaps 

 not so frequently. In the cystoliths of Ficus elastica (Fig. 88) so much 

 calcium carbonate is deposited that it effervesces with hydrochloric 

 acid. In many plants, as, for instance, most of the Characeae, the 

 quantity of calcium carbonate in their cell walls is so great as to 



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