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Special Morphology and Classification. — 249 
a single apical cell. True roots provided with a root-cap 
are also altogether wanting. 
The cell-walls usually consist of ordinary unlignified 
cellulose, which is coloured blue by iodine and. sulphuric 
acid. It is worthy of note that the cell-walls of Algze have 
a great disposition to become converted into a mucilaginous 
substance, causing the entire organism sometimes to become 
surrcunded by a gelatinous envelope, while in other cases a 
stalk-like mass of jelly connects the Alga with others of the 
same kind, or fixes it to its substratum. Many are cal- 
careous, and the calcium carbonate is, in some cases, as 
Corallina, deposited in the cell-wall ; while in others, as 
Cymopolia, it is excreted into the intercellular spaces. The 
cell-walls of the Diatomacez form, in 
consequence of the deposition in them 
of silica, actual coats of mail which are 
not destroyed either by decay or at a 
red heat. Although the Diatoms are at 
most o°I mm. in diameter, these sili- 
ceous coatings form strata which are 
not without importance in building up 
the crust of the globe, as in the case of 
emery and infusorial earth (Fig. 38, 
p. 22, and Fig. 381). 
It is rare for the cells of Algz to 
possess a nucleus ; but they often con- 
tain starch. The presence of chloro- 
phyll is of special importance ; and this 
is invariable, although often concealed 
by pigments of various colours. These 
s ; Fic. 381. — Grammntato- 
combinations of colours are remarkably —piora sublilissima, a 
constant in large natural groups. Thus Hes hee ee 
the Nostocacez are of a bluish or light- 
green colour, the Diatomacez yellow, the Fucaceze brown or 
olive, and the Floridez of a beautiful rose-red. Both the 
chlorophyll and the other colouring matters sometimes form 
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