THE LILY 161 



and, increasing in size, gradually round off and separate from one 

 another. 



2. Synapsis, a very common stage in which the chromatin within the 

 nucleus of the spore mother cell will be found collected in a dense 

 mass, generally near the nucleolus at one side of the nucleus. 

 Synapsis is a very important stage, since it appears to he charac- 

 teristic of the time when the sporophyte number of chromosomes is 

 reduced by half to the number of the gametophytes (see Principles, 

 Sec. 334). The gametophyte number of chromosomes in the lily is 

 twelve, and first appears in the nuclear divisions within the pollen 

 mother cell and the embryo sac (D, 3). 



8. The first nuclear division, or mitosis, where a large spindle will 

 be found within the spore mother cell, and the chromosomes will 

 be either at the center, forming the equatorial plate (see Principles, 

 Fig. 302, B), or separated into two groups of daughter chromosomes 

 that pass to the poles of the spindle. 



4. Two daughter nuclei in a resting condition following the first mitosis 

 and the division of the spore mother cell into two daughter cells. 



5. The second nuclear division, or mitosis, in which two spindles are 

 formed simultaneously in the two daughter cells, resulting in four 

 daughter nuclei. 



6. The final division of the spore mother cell into four daughter cells, 

 forming a tetrad, each of which becomes a pollen grain. 



The processes of pollen formation are identical in all essentials with those 

 of spore formation in the bryophytes and pteridophytes, and show that the 

 pollen grains are microspores, — a conclusion sustained by their further de- 

 velopment into reduced male gametophytes. The pollen sac is therefore a 

 microsporangium and the stamen a microsporophyll. Pollen formation in the 

 lily and related types is an attractive subject for the study of nuclear and 

 cell division in the higher plants. 



C. The pistil of the lily. Draw the pistil in side view, showing : 



1. The ovule case, or ovary, below ; the style above, bearing the three- 

 lobed stigma, or receptive region, for the pollen. Note that the ovule 

 case is three-angled, and the position of the angles with reference 

 to the lobes of the stigma. 



2. Cut sections of the ovule case both from flowers which have recently 

 opened and from those whose perianth has been withered several 

 days. Under l.p. note the three hcules, or chambers, of the ovule 

 case and the ovules within them. Show their position in an outline 

 drawing. 



The three locules of the ovule case and the three lobes of the stigma 

 present evidence that three carpels are involved in the formation of this 



