THE SPERMATOPHYTES 475 
number continues into the sperm nuclei. Each pollen grain, 
after the tetrads have separated into their components, consists of 
an outer firm wall or exosporium, an inner wall or endosporium, 
within which will be found the region called the fovilla, which is 
nothing other than a mass of protoplasm containing a nucleus. 
Before the pollen is shed from the anther, its protoplasmic 
contents undergo a series of changes leading up to the develop- 
ment of the male gametophyte. The nucleus and protoplasm 
enveloping it divides to form two cells, one a generative-cell 
A 
Stomiumy | 
Mechanical Cells, 
Endothectum ~ 
Broken Down Partition ~ 
between Miucrosporangia 
Generative Cell-~ es 
Fh 
Tube Cell-~ 
Fic. 357.—A, cross section of dehisced anther of a lily (Butomus). The layer of 
cells beneath the epidermis (exothecium) acts as mechanical tissue to open the 
pollen sacs. (After Sachs.) B, Section of a pollen grain, diagrammatic. (Mottier.) 
containing a generative nucleus, the other a tube cell containing 
a tube nucleus. The generative nucleus then divides to form 
two sperm nuclei and the partition wall between the two cells 
disappears. In this condition the protoplasmic contents of the 
pollen grain constitute the male gametophyte. (See Fig. 220.) 
POLLINATION AND FERTILIZATION 
The mature pollen grains are discharged from the ripened 
anther through the splitting open of its wall. ‘They are trans- 
ferred to the stigma of the pistil of another Erythronium flower 
through the agency of insects. Here they germinate, each put- 
