LIFE HISTORY OF THE MALE FERN 67 
repair and growth. In order to be serviceable for these purposes, 
they must be changed into water-soluble products, This is 
accomplished through the activity of substances called enzymes 
which are manufactured by the cell protoplasm. ‘The process 
whereby water-insoluble foods are converted into water-soluble 
products is called DiGEsTIon. 
Gross StruecTWRE AND HisroLocy OF THE SORI AND Spo- 
RANGIA.—The sporangia or spore cases are found clustered together 
in circular groups on the under surface of the pinnules nearer the 
mid-vein than the margin. Each group of sporangia is covered» 
Fic. 36.—Sporangia of an undetermined species of fern; /, 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.) 
with a membranous expansion of the epidermis called the 
indusium. ‘The whole is called a sorus (Fig. 28A) (pl. sori) and 
contains many sporangia. Each sporangium is composed of: 
(a) the stalk of considerable length and usually comprising three 
rows of cells, outgrowths of the epidermis of the pinnule; and (6) 
the head; sub-globular and hollow, consisting for the most part of 
a covering wall of thin-walled, flattened cells, within which will 
be noted a marginal ring of cells, with walls having U-shaped 
thickenings, and called the annulus. Beneath the wall of an 
immature head of a spore case occurs a tapetum of a single layer of 
cells and in the center spore mother cells. The spore mother 
cells undergo two successive divisions to form tetrads or groups of 
four spores which later separate as mature spores. During 
the first of these divisions reduction in the number of chromo- 
somes takes place from the number characteristic of the species 
(diploid number) to the haploid number or half aaa or 
