SPERMATOPHYTES— ANGIOSPERMS 



361 



The Belling method, as modified by Dr. McClintock, is more rapid 

 and gives good permanent preparations. 



The pollen grain at the time of shedding generally consists of two 

 cells, the tube cell and the generative cell, which afterward divides and 

 forms two male cells or two male nuclei. Lilium and Erythronium 

 furnish good illustrations of pol- 

 len shed in the two-cell stage 

 (Fig. 128). In Silphium, Sambu- 

 cus, and Sagittaria the genera- 

 tive nucleus divides before the 

 pollen is shed. The division of 

 the generative cell to form the 

 two sperms takes place just be- 

 fore fertilization; consequently, 

 in forms like Silphium, fertiliza- 

 tion is Hkely to occur within less 

 than 72 hours after the division 

 of the generative cell. 



Sections should not be more 

 than 5 m thick, if they are to 

 show a clear differentiation of 

 exine, intine, starch, and other 



structures. If sections have been 



stained in iron-haematoxyhn, 



staining 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 2-5 per 



cent solution of cane-sugar in water. Where the interval between pol- 

 lination and fertilization is known (about 72 hours in Lilium phila- 



delphicum and 96-100 hours in L. canadense) , pieces of the stigma and 



style showing pollen tubes can be selected with some certainty. 



Fig. 128. — Erythronium americanum: photo- 

 micrograph of mature pollen grains; the one at the 

 top, which is cut longitudinally, shows both the 

 tube nucleus and the conspicuous generative cell; 

 the other is cut transversely and shows the gen- 

 erative cell, but not the tube nucleus. Stained in 

 safranin and gentian violet; from a preparation by 

 Dr. Lula Pace. Cramer contrast plate; 4-mm. ob- 

 jective; ocular X4; yellowish -green filter; bellows, 

 85 cm.; exposure, 3 minutes. X615. 



