92 ALDRIDGE ON POLLEN. 



water, a coloured gelatinous slime, which suffered no altera- 

 tion by iodine: this is only to be accurately distinguished 

 from the other constituents of the fovilla, in the Najas major. 

 Yet he thinks its occurrence is universal, and that it agrees in 

 its characters with the inner membrane of the pollen, so that 

 it is not improbable, that it furnishes the material for the 

 hoyau; [i. e. the imaginary membranous tube, continuous 

 with the inner membrane, and growing at the expense of the 

 stigmatic fluid. ^ 



The second, and never absent constituent of the fovilla, 

 is the oil, which lies distributed in very small drops, through 

 the whole mass of the mucus. We recognise it, by its suffer- 

 ing no alteration by iodine, when by immersion in water the 

 fovilla becomes protruded. We can frequently convince 

 ourselves of the existence of these little oil-drops, by treating 

 with a diUited acid, whereby we can observe an evident 

 flowing together of the sanse. Besides, we convert the 

 very little oil-drops into large drops flowing together, when 

 we gently stir dried pollen placed in water. 



The third constituent of the fovilla is the easiest to recog- 

 nise — namely, the starch : it is found in grains of different 

 sizes and forms, and is sometimes present in very great quan- 

 tity, but appears to be permitted to be at other times wholly 

 absent. Generally the starch-grains, as well as the oil-drops of 

 the same form, are distributed in the whole mass of the par- 

 ticular mucus, and the Najas major constitutes the only ex- 

 ception. We can best perceive theexistence of the starch- 

 grains, when we press the pollen between two glass plates, 

 and pour on it a little solution of iodine, without stirring the 

 preserved mixture afresh. The starch-grain shows itself then 

 with the blue colour of iodide of starch, which we, by this pro- 

 ceeding, can at once perceive. We may treat the pollen fij'st 

 with acid, and 'then bring in contact with the solution of 

 iodine; while a part of the fovilla is being forced out, then 

 begins round the expelled and coagulated mucus from which 

 the acid has liberated some starch, the blue cloud by which 

 the iodide of starch exhibits itself; and by degrees the whole 



