SPERMATOPHYTES— GYMNOSPERMS 



327 



to remove a small piece of the nucellus 5-7 mm. square, according to 

 the species. Fix in chromo-acetic-osmic acid (1 g. chromic acid, 2 c.c. 

 acetic acid, 6 c.c. 1 per cent osmic acid to 100 c.c. water). Nothing sur- 

 passes iron-alum haematoxylin for all the stages in the development of 

 the male gametophyte (Fig. 112 A-C). By using a 2 per cent iron- 

 alum for 5 or G hours and staining overnight, or even 24 hours, the 

 stain can be drawn so precisely that the portion of the cilia between 



Fig. 112. — Ceratozamia mexicana: A, pollen grain which has been in a sugar solution for two 

 days; X876. B, nucellus with numerous pollen tubes; X 17. C, basal end of pollen tube showing the 

 persistent prothallial cell; outside it the stalk cell; and, above, the two sperms still inclosed in the 

 sperm mother-cells; X156. 



the blepharoplast and the surface can be differentiated from the free 

 portion (Fig. 113). 



The pollen tubes, with their sperms, make instructive preparations 

 when mounted whole. Fix the nucellus, with its pollen tubes, as if for 

 paraffin sections. About 6 per cent formalin in water has proved suc- 

 cessful. Wash in water for 2 hours and stain in aqueous safranin, 4 or 5 

 hours. Extract the stain until it is satisfactory, and then transfer to 



