144 MORPHOLOGY OF THE ANGIOSPERMS 



sporogenous cells formed in tliis way may persist together, even to the 

 mature pollen stage. Large, so-called placentoids are merely lobes of 

 connective tissue, not division walls. The sporangial position where all 

 four sporangia lie in one plane, as in Sassafi'as and other lauraceous 

 genera, suggests placentoid division of two sporangia but is the result 

 of major displacement of sporangia by differential growth in the de- 

 veloping anther. Placentoids occur in rather few taxa, chiefly in 

 Labiatae, Acanthaceae, Bignoniaceae, Solanaceae, Scrophulariaceae. 



Ontogeny of Spore Mother Cells. The primary sporogenous cells may 

 differentiate directly as spore mother cells — many Compositae, Labiatae, 

 Malvaceae, Portulacaceae — or may divide, usually sparingly, and their 

 daughter cells become spore mother cells. Where division does occur, 

 the adjacent tapetal cells may also divide, maintaining the tapetal 

 sheath about the enlarging cell mass. The spore mother cells (here 

 "pollen mother cells") rapidly increase in size. The nuclei enlarge, the 

 cytoplasm becomes dense, and the walls thicken. Wall thickening is 

 uneven in cells which form pollen grains that remain in tetrads or 

 larger clusters; the outer walls of the cluster are thicker than the 

 internal ones. 



Microspore Formation. Microspores are formed from the spore mother 

 cells in tetrads by two closely successive divisions, the meio'tic or re- 

 duction divisions, during which the chromosome number is reduced 

 from 2n to n. Meiosis is nearly simultaneous throughout a sporangium 

 or anther sac, except in very long anthers, as in Liriodendron, where 

 there is an acropetal succession in stages of pollen development. Meiosis 

 follows two somewhat different methods: the simultaneous method, 

 where no wall is formed after the first division, the two divisions being 

 almost simultaneous, with four free nuclei in the mother-cell protoplast; 

 and the successive method, where a wall is formed after the first di- 

 vision, and each of the cells so separated is then divided. The simul- 

 taneous method is characteristic of dicotyledons and the successive, of 

 monocotyledons, but there are many exceptions. There is probably little 

 or no phylogenetic significance in these differences. Cell division in 

 spore formation also takes place by cell-plate formation or by furrow- 

 ing, sometimes called constriction, which occurs by means of the divi- 

 sion of the protoplast by the extension inward from the mother-cell wall 

 of wedges of new wall, as the protoplast contracts after nuclear 

 division. 



There seems to be much variety in relationship of mother-cell wall 

 and the walls of the four spores. The mother-cell wall may remain in- 

 tact, even until the pollen grains are mature, enclosing them when they 

 are shed as clusters, or may disappear early or late in pollen matura- 

 tion. Where the spores remain together as compound pollen grains and 



