SPERMATOPHYTES— ANGIOSPERMS 357 



than 4 mm. in diameter, cut out the cluster, leaving only enough 

 tissue at the base to hold the buds in place. Larger heads should be 

 cut separately. 



Our most common thistle, Cirsimn lanceolatum, shows the floral de- 

 velopment with unusual clearness, but the preparation of the material 

 is somewhat tedious. The involucre, which is too hard to cut, must be 

 carefully dissected away. Retain only enough of the receptacle to 

 hold the developing florets in place. A series of sizes with disks vary- 

 ing from 3 to 10 mm. in diameter will show the development from the 

 undifferentiated papilla up to the appearance of the archesporial cell 

 in the nucellus of the ovule. The Canada thistle, Cirshwi arvense, is 

 equally good, but it is more difficult to dissect out the desirable parts. 

 In the common sunflower, Helianthus annuus, the young floral parts, 

 like the mature head, are so very large that a satisfactory study may 

 be made with a low-power objective. As in the case of the thistle, the 

 involucre must be trimmed away and only enough of the receptacle 

 retained to hold the florets together. 



Erigeron philadelphicus furnishes a beautiful example of epigynous 

 floral development, and the heads are so densely clustered that, in a 

 single section, one may find various stages from heads with undiffer- 

 entiated disk up to heads with florets showing pappus, corolla, sta- 

 mens, and carpels (Fig. 126). In the Chicago region, the last two 

 weeks in May are best for these stages. 



Spermatogenesis. — The earlier stages in spermatogenesis will be 

 found in the preparations of floral development. The origin of the 

 archesporium, the origin of sporogenous tissue, and the formation of 

 the tapetum are beautifully shown in longitudinal and in transverse 

 sections of the anthers of Taraxacum and many other Compositae. 

 Transverse sections of the head of Taraxacum, or any similar head at 

 the time when pollen mother-cells are rounding off in the center of the 

 head, will show various stages from the mother-cells in the center to 

 the tetrads of spores at the periphery. Transverse sections of the 

 anther of Polygala give exceptionally well-defined views of the arche- 

 sporial cells and sporogenous areas. 



Lilium, Trillium, Galtonia, Iris, Tradescanlia, Vicia, and Podo- 

 phijllum can be recommended for demonstrating the nuclear changes 

 involved in the formation of spores from the mother-cell (Fig. 127). 

 Several species of Lilium are common in greenhouses, and these may 

 be used where wild material is not available. In early stages, where 



