246 ELEMENTS OF BOTANY 
a gentle stream of water from a wash-bottle. The student should 
then mount the prothallium, bottom up, in water in a shallow cell, 
cover with a large cover-glass, and examine with the lowest power 
of the microscope. Note: 
(a) The abundant root-hairs springing from the lower surface of 
the prothalliuin. 
(v) The variable thickness of the prothallium, near the edge, 
consisting of only one layer of cells. 
(c) (In some mature specimens) the young fern growing from 
the prothallium, as shown in Fig. 172, B. 
The student can hardly make out for himself, without much 
expenditure of time, the structure of the antheridia and the arche- 
gonia, by the coépera- 
tion of which fertili- 
zation takes place on 
much the same plan 
as that already de- 
scribed in the case of 
mosses. The fertil- 
ized egg-cell of the 
archegonium gives 
rise to the young 
fern, the sporophyte, 
which grows at first 
at the expense of the 
parent prothallium, 
but soon develops 
roots of its own and 
Fig. 172. Two Prothallia of a Fern (Aspidium). leads an independent 
A, under surface of a young prothallium; ar, arche- existence. 
gonia; an, antheridia; 7, rhizoids. B, an older 316. Nutrition. — 
prothallinm with a young fern-plant growing from 
: oe The mature fern 
it; 2, leaf of young fern. (Both X about 8.) . Et 
inakes its living, as 
flowering plants do, by absorption of nutritive matter from the soil 
and from the air, and its abundant chlorophyll makes it easy for the 
plant to decompose the supphes of carbon dioxide which it takes in 
through its stomata. 
