336 FERNS AND THEIR I ALLIES 
and soon begin to grow. The sprout from the . spore is 
in the form of a single thread which is a protonema. From 
the fern protonema there develops a small, flat, heart- 
shaped body called the prothallium (Greek, pro, before; 
thallos, twig) which is indispensable to the life of the fern. 
On the under surface of the prothallium grow small bodies, 
the antheridia and archegonia. The 
antheridia produce numerous motile 
sperm cells, and each archegonium a 
single egg cell. A sperm cell, swim¬ 
ming about on the surface of a pro¬ 
tonema when it is wet, is attracted 
to an archegonium by a substance 
which is formed on it. This it enters, 
fuses with the egg cell, and forms the 
fertilized egg cell. Pro thallium is the 
name of the fern gametophyte. (See 
section 281.) 
When an egg cell is fertilized, it 
begins to divide and a new fern plant 
is soon formed. The young plant 
remains attached to the prothallium 
and gains nourishment from it. As 
soon as the young fern is able to get 
nourishment by its own roots, it 
begins life as an independent plant 
and the prothallium dies. There is 
the same alternation of generations 
in the fern that occurs in the mosses, the prothallium 
being the gametophyte and the “fern” the sporophyte, 
but the latter is the longer lived and much the larger plant 
(Figure 305). The prothallium is so small, in fact, that 
it is seldom noticed, while in the life history of mosses the 
green, leafy gametophyte is larger than the sporophyte 
and lives longer. 
Figure 308. — Equisetum, 
Fertile Stalk. 
A , sporangial cone ; B, 
collar of teeth; C, node; 
D, furrow; E, ridge. 
