3<5 



MORPHOLOGY OF THE CELL. 



of swelling, appear, in the presence of a small quantity of water, as a hyaline layer 

 enveloping tiie seed ; with more copious addition of water they become more and more 

 diluted into thin mucilage. A similar process occurs in some other seeds, as those 

 of Teesdalia nudicauUs and Plantago Psyllium, in the seed-hairs of Ruellia, and the 

 pericarp of Salvia. Gum-tragacanth consists of the cells of the pith and medullary 

 rays of Astragalus creticus, A.lragacanthayTund other species, transformed into mucilage^ 

 When the walls of these cells become mucilaginous, and swell up on copious addition 

 of water, they force themselves through cracks in the stem as viscid masses, and dry 

 up on the outside into a horny substance capable of swelling in water. Vegetable 

 mucilage can, however, arise in other ways ^. 



(f) Incombustible Deposits occur in every cell-wall. The presence of lime and silica 

 can be directly proved, and it can scarcely be doubted that potash, soda, magnesia, 

 iron, sulphuric acid, &c., also occur in small quantities ; the lime-salts and silica 

 increase with age. The deposition may take place in two ways. Usually only ex- 

 tremely small particles of incombustible substance are deposited regularly between 



the molecules of the organised substance of 

 the cell-wall ; and this may be recognised by 

 the ash remaining behind after ignition in 

 the form of the organised cell-wall (as a 

 skeleton). But lime-salts may also be con- 

 tained in the cell-wall in the form of nu- 

 merous very small crystals; they then lie 

 imbedded in the substance of the cell- 

 wall itself, sometimes in the form of 

 growths which project into the cell-cavity 

 {Cyjtoliths). 



Skeletons composed of a substance^ soluble 

 in weak acids (generally thought to be lime) 

 are obtained by combustion of very thin 

 layers of tissue on glass or platinum-foil ; 

 they occur so generally that it is unneces- 

 sary to adduce examples ; from entire vascular cells I obtained, in the case of Cucurbita 

 Pepo, beautiful lime-skeletons. Silica-skeletons are obtained most abundantly from epi- 

 dermal cells and from Diatoms ; but silicified cell-walls occur also in the interior of 

 tissues, as in the leaves of Ficus Sycomorus, Fagus sylvatica, Quercus suber, Deutzia scabra, 

 Phragmites communis, Ceratonia Siliqua, Magnolia grandijlora, &c., according to Mohl*. 

 The silicification does not generally affect the whole thickness of the cell-wall, but 

 only an outer shell ; as, for instance, in the case of epidermal cells, the cuticularised 

 portion only. In order to obtain fine skeletons, it is necessary previously to soak the 

 removed epidermis or thin sections of it in nitric or hydrochloric acid, and then to 

 burn them on platinum-foil. I have found another method much more convenient. 

 I place larger pieces of the tissue {e.g. of leaves of grass, stems of Equisetum, &c.) on 

 platinum-foil in a large drop of concentrated sulphuric acid, and heat over the flame ; 



Fig. 39.— Section of'the endosperm of Ceratonia 

 Siliqua. 



^ [H. von Mohl, Bot. Zeitg. 1857, p. 33 ; Pharmaceut. Journ. Jan. 1859.] 



2 Compare further, Frank, Ueber die anatomische Bedeutung und die Entstehung der vegeta- 

 bilischen Schleime, in Jahrb. fiir wissen. Bot. vol. V. 1866. 



^ The salts found in the ash are partly products of combustion. Carbonates may be pro- 

 duced by the combustion of salts of vegetable acids. Since a strong red heat is necessary 

 for complete combustion, easily volatile chlorides (potassium or sodium chloride) may disappear 

 from the ash. 



* H. von Mohl, Ueber das Kieselskelet lebender Pflanzenzellen, in Bot. Zeitg. 1861, no. 30 et 

 seq. — Rosanoff, Bot. Zeitg. 1871, nos. 44, 45. 



