158 ORGANOGRAPHY. BOOK I. 



under water, in which the coating is certainly simple. That 

 the pollen-tube itself has not been found by some observers, 

 has probably arisen from its having been looked for in pollen 

 made to burst on the field of the microscope, immersed in 

 water, when the pollen-tube is scarcely ever emitted. The 

 vital action that causes the emission seems to depend upon 

 contact with the secretion of the stigmatic surface. The 

 pollen-tube is, therefore, only to be sought in pollen that 

 lias been some time upon the stigma. 



The colour of pollen is chiefly yellow. In Epilobium an- 

 gustifolium it is blue, in Verbascum it is red, and it occa- 

 sionally assumes almost every other colour, except green. 

 According to Fourcroy and Vauquelin, the pollen of the Date 

 tree consists of malic acid, phosphate of magnesia and lime, 

 and also of an insoluble animal matter intermediate between 

 gluten and albumen. Macaire Prinsep has ascertained that 

 the pollen of the Cedar contains malate of potass, sulphate of 

 potass, phosphate of lime, silica, sugar, gum, yellow resin, and 

 a substance which, by its characters, approximated to starch. 

 Being analysed as a whole, it gave, per cent., 40 carbon, 

 11 "7 hydrogen, and 48*3 oxygen, but no nitrogen. — Bill. 

 Univers. 1830. 45. 



The matter contained in the granules is called the fovilla. 

 Under common magnifiers it appears like a turbid fluid; 

 imder glasses of greater power it has been found to consist of 

 a multitude of particles moving on their axes with activity, of 

 such excessive minuteness as to be invisible, unless viewed 

 with a magnifying power equal to 300 diameters, and mea- 

 suring from the 4000th or 5000th to the 20,000th or 30,000th 

 of an inch in length. This motion was first distinctly noticed by 

 Gleichen ; but it seems to have escaped the recollection of 

 succeeding botanists until the fact was confirmed by Amici, 

 who some time before 1824 saw and described a distinct, 

 active, molecular motion in the pollen of Portulaca oleracea. 

 In 1825 the existence of this motion was confirmed by Guille- 

 min, who ascertained its presence in other species. In June 

 1827 I was shown the motion by Dr. Brown, who subse- 

 quently published some valuable observations upon the subject, 

 without however noticing those of either Amici or Guillemin. 



:i 



