Spermatophytes -Angiosperms 



273 



and forms two male cells or two male nuclei. Lilium and Ery- 

 thronium furnish good illustrations of pollen shed in the two-cell 

 stage (Fig. 95). In Silphium, Sambucus, and Sagittaria the genera- 

 tive nucleus divides before the pollen is shed. 



Sections should not be more than 3 to 5 JJL thick, if they are to 

 show a clear differentiation of exine, intine, starch, and other struc- 

 tures. If sections have been stained in iron-haematoxylin, staining 



--.> -3V-:..-. 



.- J* . -v_ ^ * , 



&~5^$$ W Jtiflfe^i 

 JigS^ : > 'I^W^ =1 



^^Si^!i* .' ; -B^^r^K*-*- 



; ^>ft^r'A : 'r'.'5^S*ti'Sj>"' 





FIG. 97. Lilium philadelphicum: photomicrograph of transverse section of ovary 

 showing, in one of the ovules on the left, the first mitosis in the megaspore mother cell; 

 and, in one of the ovules on the right, the second mitosis which gives rise to the four 

 megaspore nuclei chro mo-acetic acid; safranin, gentian- violet, orange. Cramer con- 

 trast plate; 16 mm. objective; ocular X4; yellowish-green filter and also a strong filter 

 such as is used in outdoor work; camera bellows, 30 cm.; exposure, 2 minutes. X64. 



in safranin for from 3 to 7 minutes will give the exine a bright-red 

 color and will not obscure the haematoxylin. A rather sharp stain 

 in gentian-violet will stain the starch and also the intine. In 

 Asclepias and many orchids, in which a common exine surrounds the 

 entire mass of pollen grains, care must be taken not to overstain. 



In many cases the pollen grains will put out their tubes in a 2 to 

 o per cent solution of cane-sugar in water. Where the interval 



