THE POLLEN-GRAIN. 



391 



carbolic acid. In a word, the most favourable clearing medium for 



each object must be found by experiment. Upon the preparation 



in oil of lemon or in chloral 



hydrate we can determine, 



by optical sections, the v^y^^**^ / 



A 



position of the valve inside 

 the exine, in which it is 

 found wedged with its base 

 somewhat broadening in- 

 wards (Fig. 143, B). Under 

 the valve can be seen the 

 bulging of the intine. In 

 sulphuric acid the oil-drops 

 on the exine become blue, 

 and the exine itself slowly 

 becomes brown. The valve 

 is thrust off by the swelling 

 contents. In 25 per cent 

 membrane is soon dissolved ; 



FIG. 143. A , entire pollen-grain oiCiicui-bita 

 J J ep<>, seen in surface view, and partly also in 

 optical section ; taken from a preparation in oil 

 of lemon (x 240). B. pollen-grain of Oucurbita 

 verruousa ; fragment of a cross-section through 

 the membrane ( x 540). 



chromic acid the entire pollen- 

 the intine resists somewhat longer, 

 and, at the moment when the exine disappears, can be traced as a 

 strongly swollen, homogeneous membrane. The pollen-grain has 

 previously emptied itself, whereby the observation of the intine is 

 considerably facilitated. In sulphuric acid, on the other hand, the 

 intine is immediately dissolved, the exine remains, the extruded 

 contents of the pollen-grain are gradually, as in other cases, 

 coloured rose-red. 



Compound Pollen-Grains. Of compound pollen-grains, which 

 occur alike in Monocotyledons and Dicotyledons, we will select 

 those of the Ling, Calluna mlyaris. The grains here are joined 

 in fours, and usually grouped tetrahedrally. The pollen-mem- 

 brane shows only slight protuberances, and usually three exit- 

 places for each grain. The Various species of Erica, Azalea and 

 Rhododendron agree in all essentials with Calluna. In species of 

 Acacia, as in the Mimoseae generally, the pollen-grains form 

 groups of four, eight, twelve and sixteen, and even more cells, 

 but can occur also separately. 



Artificial Culture of Pollen- Grains. The artificial production 

 of pollen-tubes is as a rule very easy. In a from 3 to 30 per 

 cent, solution of sugar, in spring (not distilled) water, to which 

 1-5 per cent, gelatine has been added, most pollen-grains readily 

 put out their tubes, in which protoplasmic movement is beautifully 



