GROUPS OF PLANTS. 85 



external layer of cells, which constitutes the exothecium (Fig. 53, 

 ex). These four sporangial regions may remain more or less 

 distinct and separate at maturity, or the two on either side niay 

 coalesce. This latter usually occurs at maturity, when dehiscence 

 takes place, forming apparently a single pollen sac on either side 

 of the connective or axis (Fig. 53, H). 



The Microspores (pollen grains) are developed somewhat 

 differently in Monocotyledons and Dicotyledons. In most mono- 

 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 (daughter cells) in turn 

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

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

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



Fig. ss. Development of male gametophyte in an Angiosperm. I, pollen grain 

 (microspore) which has divided into the mother or generative cell (v) and a larger tube-cell 

 with nucleus (sk); II, appearance of pollen grain on treatment with osmic acid show- 

 ing the separation of the generative cell (v) from the wall of the pollen grain; o, at the 

 right giving a view of the generative cell with the nucleus embedded in the hyaline proto- 

 plasm; III, showing the development of the tube-cell into the pollen tube which 

 contains the two male cells (nuclei) or gametes formed by the generative cell. — ^After 

 Elfving. 



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

 called (page 49) 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 germ- 

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

 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 Ijetrads may be more or less agglutinated, as in the 

 orchids and milkweeds, these masses being known as pollinia. 



