PRINCIPAL GROUPS OF PLANTS. 



123 



cotyledons the nucleus of each cell (pollen mother cell) making 

 up the archesporium divides into two nuclei, each of which takes 

 on a wall of cellulose. Each of these (dau^^hter cells) in turn 

 divides, giving rise to four pollen grains. In dicotyledons (Fig. 

 80) the nucleus of a mother cell divides into four nuclei before 

 the walls are formed which separate the nuclei, thus giving rise 

 to the tetrad group of spores to which attention has already been 

 called (Fig. 60, D) under Bryophytes. The wall of each spore is 

 divided into two layers, an inner layer consisting of cellulose 

 known as the intine, which gives rise to the pollen tube on germi- 

 nation of the spore ; and an outer layer somewhat different in 



Fig. 80. Development of pollen grains (microspores) of garlic (.Allium narcissiflorum): 

 a, pollen mother cell with nucleus; b, the same with homogeneous nucleus and a thicker 

 wall; c-e, changes in nucleus prior to division; f, formation of spindle with nuclear masses 

 in the center from which nuclear threads extend to the poles of the spindle; g, division of 

 nuclear substance and receding of it from the center of the cell; h-i, further stages in the 

 organization of the nuclear substance at the poles; k. formation of a wall between two 

 daughter cells; 1, beginning of division of one daughter cell; m-n, final divisions resulting 

 in the formation of a tetrad (group of 4 cells). — After Strasburger. 



composition and variously sculptured, known as the exine. When 

 the spores are mature the original walls of the cells of the arche- 

 sporium dissolve and the ripe pollen grains are set free, forming 

 a yellowish powdery mass filling the pollen sac. In some cases 

 the spores of the tetrads hang together, or even the whole mass 

 of pollen tetrads may be more or less agglutinated, as in the 

 orchids and milkweeds, these masses being known as poUinia. 

 Male Gametophyte. — Before the dispersal of the pollen grains 

 or microspores, certain changes leading to the development of 

 the gametophyte have taken place (Fig. 8i). The spore, as we 

 have seen, is unicellular. This divides into two cells : one, which is 

 relatively small, known as the mother cell of the antheridium 

 (Fig. 81, v), and another, which, composed of the remaining 



