146 PLANT ANATOMY. 
inhabitants of the sea; fluorine in the testa of the seed of varie- 
ties of grain, and lithium in tobacco. 
As silicium has in recent times been introduced into organic 
compounds in the place of carbon, the supposition is not entirely 
unjustifiable that the silicium contained in the cell-wall may be 
present in the form of an organic compound.* 
As is already naturally evident from mechanical principles, 
an increased thickening and solidity of the cell-walls corresponds 
by no means to a greater amount of incombustible substances.’ 
The delicate tissue, containing air, of peeled colocynth, dried at 
100° C., afforded 11 per cent of ash, the seed only 2.7 per cent. 
Quassia wood from Surinam yields 3.6 per cent, the bark 17.8 
per cent of ash; guaiac wood, which is so exceptionally dense, 
and which consists almost exclusively of strong wood-cells, gives, 
nevertheless, scarcely 1 per cent of ash. Leaves very frequently 
contain more than 10 per cent of inorganic constituents, for 
instance, Folia Stramonii as much as 17, and tobucco leaves 
occasionally 27 per cent of the substance dried at 100° C. 
The developing or merismatic, and the assimilating tissues 
(cambium, mesophyll of leaves) and organs (leaves* and barks) 
are richer in ash than the completely developed and non-assimi- 
lating (wood). The mineral substances migrate from the fin- 
ished tissues to the places where development is going on. It 
is also seen from this that they must play an important part in 
the formative processes of plants.‘ 
To obtain the ash in a condition suitable for weighing is often 
somewhat difficult, from the fact that many parts of plants, 
and especially secreted substances, as gum, resin and sugar, 
undergo complete combustion only very gradually. The incin- 
eration of such substances, especially of those rich in nitrogen, 
'“ Berichte d. Deutsch. Chem. Ges.,” 1872, 568. 
*That the strength of flexure, for instance, of the grasses, is entirely 
independent of the silica contained therein, is shown by rematmieanens 
in solutions free from silicium, 
* About the time of the falling of the foliage the leaves become con- 
mabey poorer in mineral substances. 
‘This is also evident from the experiments with solutions of nieve 
_ substances, 
