3« 
MORPHOLOGY OF THE CELL, 
of swelling, appear, in the presence of a small quantity of water, as a hyaline layer 
enveloping the 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 nudicaulis and Plantago Psyllium, in the seed-hairs of Ruellia, and the 
pericarp of Sal'via. Gum-tragacanth consists of the cells of the pith and medullary 
rays of Astragalus creticus, A. Tragacantha, and 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 mt^iivilos» 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 
{Cystoliths). 
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 syl'vatica, Quercus suber, Deutzia scabra, 
Phragmites communis, Ceratonia Siliqua, Magnolia grandißora, &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; 
' [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. für 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. 
Fig. 39. — Section of the endosperm of Ceratonia 
Siliqiia. 
