PLANT ORGANS AND ORGANISMS 193 



within. Each spore mother-cell undergoes division and redivision 

 into four spore daughter-cells, at the same time that reduction in the 

 chromatin substance takes place in these cells. Thus originate 

 tetrads (groups of four) of spore daughter-cells inside spore mother-cell 

 wall. These continue to enlarge, press against the mother-cell wall 

 which becomes converted into mucilage and each of the tetrad cells 

 becomes in time a mature microspore or pollen grain. 



During this time the entire anther is growing in size, the cells of 

 the endothecium in one or more layers becomes thickened by lignin 

 deposits to form a mechanical endothecium', the tapetum gradually 

 breaks down and appears only , at length, as an irregular layer around 

 the maturing pollen cells. When the anther is finally ripe the parti- 

 tion between each pair of microsporangia becomes narrowed, flat- 

 tened and ruptured and thus numerous microspores or pollen grains 

 fill two cavities, one on either side of the connective. The micro- 

 spores or pollen grains at first show only a thin clear cellulose layer, 

 but from this, by a differentiation of the exterior film, the exospore 

 layer becomes cut off. This becomes cuticular. The cellulose inner 

 layer (endospore), remains unaltered. In the development of the 

 exopore, one to several deficiencies are usually left in it through 

 which the endospore may protrude later as the rudiment of the 

 pollen tube. 



Attachment of Anther. The attachment of the anther to the fila- 

 ment may be in one of several ways, as follows : 



Innate, attached at its base to the apex of the filament. 



Adnate, adherent throughout its length. 



Versatile, when the anther is attached near its center to the top 

 of the filament, so that it swings freely. The adnate and versatile 

 are introrse when they face inward, extrorse when they face outward. 



Pollen. The pollen grains or microspores vary in form for differ- 

 ent species and varieties and while they are averagely constant for 

 these, nevertheless many exceptions have been recorded. The* fol- 

 lowing are some of the commoner forms : 



Four Spore Daughter-cells, hanging together as in the Cat Tail 

 (Typha) forming a pollen grain. 



Elongated, simple pollen grains as in Zostera. 



Dumb-bell-shaped, as the pollen of the Pines. 



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