CHAP. II. 
POLLEN. 
191 
tinguisliable from the moving, ultimate, organic molecules 
common to all parts of a vegetable ; the other, much larger, 
often oblong, and unlike any other kind of particle hitherto 
detected in plants. Clarkia pulchella, and some other Ona- 
graceous plants, show this difference, as well as the motion, in 
a very conspicuous manner. In consequence of their manifest 
motion it has been conjectured that the larger particles of 
the fovilla were the incipients of the embryo, and that it is 
by the introduction of one or more of these into the ovule that 
the act of impregnation is accomplished by the deposit of a 
rudimentary embryo in the ovule. But both Fritzsche and 
Mohl agree in considering many of the smaller particles of 
the fovilla as minute drops of oil : the molecular motion has 
been ascribed to currents in the fluid, in which the fovilla is 
suspended, and which, according to Fraunhofer, no precau- 
tions can possibly prevent; and, what is more important, the 
larger particles become blue upon the application of iodine, 
without however losing their property of motion, as Fritzsche 
has shown : they are therefore starch. 
When the pollen falls upon the stigma it emits a fine 
transparent tube, which is a prolongation of the intine, and 
down which the fovilla passes until the grain is emptied. The 
pollen-hibe thus formed was first observed by Amici, and is 
now known to be constantly produced at the period of impreg- 
nation. Of the important offices these tubes have to per- 
form an account will be given in Book II. Chap. vi. 
For further information concerning pollen the reader is 
referred to the following works : — 
I. Ynixsche, De Plantarum Polline : Berolini, 1833. This 
ingenious observer found that several modes of examining 
pollen are preferable to those usually employed: in par- 
ticular he recommends the employment of sulphuric acid, in 
the proportion of two parts of concentrated acid to three 
parts of water, for the purpose of viewing the pollen by trans- 
mitted light ; by this means it is rendered transparent, and 
the spontaneous emission of pollen-tubes is effected. In 
cases of very opaque pollen he employs oil instead of diluted 
acid, and he finds it renders an object more transparent than 
the acid itself; and in other cases, where the coat of the 
