THE POLLEN-GRAIN. 



389 



We can use the large pollen-grains of Altliaa rosea, or of Malva 

 crispa, for the purpose of preparing sections. Material hardened 

 in alcohol is best for our purpose, and we lay if before use in 

 a mixture of equal parts alcohol and glycerine. We prepare 

 a thick solution of gum, to which we add a small quantity of 

 glycerine, place a drop of this solution upon the smooth end 

 of a piece of elder-pith, and place some pollen-grains on the 

 drop. These are stirred into the gum, and then the drop either 

 allowed to dry in the air, the elder-pith being placed in a per- 



FIG. 1 42. Pollen-grains of Malta cris}>a. A , piece of a grain seen from the surface. 

 B, portion of a section through a grain. C, a pollen-grain removed from the stigma, 

 and showing pollen-tubes. D, portion of a similar grain in optical section (.4, R 

 and />. x 540. C, x 240). 



pendicular position, or else is hardened by laying for several 

 hours in alcohol. After this has happened, we prepare delicate 

 sections through the gum with a very sharp razor. The sections 

 can be small to any degree, but must be exceedingly thin. They 

 are laid in water or in dilute glycerine, whereby the gum is 

 dissolved and the enclosed sections of pollen-grains are set free. 

 In such sections the structure of the wall of the pollen-grains 

 can be studied in all its details. Such a section of Malva crispa 

 (Fig. 142, B), shows the exine to consist most externally of 



