LIFE HISTORY OF THE MALE FERN 
41 
which will be noted a marginal ring of cells, with walls having 
U-shaped thickenings, and called the annulus. 
Within the sporangium are found the spores. Each spore is a 
single cell composed of an outer brown wall with band-like markings 
called an exosporium , an inner thinner wall or endosporium, and with¬ 
in this a mass of protoplasm containing a nucleus. 
. Rupture of Sporangium and Spore Dissemination— As was pre¬ 
viously indicated, each sporangial head has a row of cells with 
U-shaped thickenings around the margin called an annulus. As the 
sporangium matures the water escapes from the cells, pulling them 
together and holding the annulus like a bent spring. The thinner 
Fig. 16. —Sporangia of an undetermined species of fern; li, lip-cells; an, annulus; 
st, stalk; sp, mature spores. Each of the four nuclei in the upper cells of the 
stalk is in the terminal cell of one of the four rows of cells that compose the stalk. 
(Gager.) 
walled cells at the side of the spore case opposite the annulus, unable 
to stand the strain, are consequently torn; the annulus then 
straightens and a wide rent is made in the sporangium. The annulus 
then recoils and hurls the spores out of the sporangium. This 
closes the sporophyte generation. 
History of the Gametophyte or Sexual Generation. —The fern 
spore, falling upon a moist surface, germinates, producing a delicate 
green septate filament called a protonema. One end of this structure 
shows larger cells, which, by the formation of oblique walls, cut out 
an apical cell of somewhat triangular shape. This is the growing 
point of what eventually becomes a mature, green, heart-shaped 
body called the “ prothallium ” or “prothallus.” The prothallium, 
about the size of an infant’s finger nail, develops on its under surface 
