CHAP. II. 
POLLEN. 
159 
The most important addition that was made by Brown to the 
knowledge that previously existed, consisted in the discovery 
of the presence of two kinds of active particles in pollen, of 
which one is spheroidal, extremely minute, and not distin- 
guishable from the moving ultimate organic molecules com- 
mon to all parts of a vegetable, the other much larger, often 
oblong, and unlike any other kind of particle hitherto detected 
in plants. 
The supposed functions of these particles will be explained 
hereafter. For the present it will be sufficient to remark, 
that some of the best subjects in which to witness their mo- 
tions are Clarkia pulchella, Mirabilis jalapa, and Lolium 
perenne. 
For the fullest information concerning the nature of pollen, 
the student should consult Fritzche de plantarum polline 
BeroUni, 1833. This ingenious observer found that several 
modes of examining pollen are preferable to those usually em- 
ployed. In particular 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 transmitted 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 dilute acid, and he finds it renders an object more trans- 
parent than the acid itself ; and in other instances, where the 
coat of the pollen is either too much or too little transparent 
to show the apertures in its sides, he finds a solution of Iodine 
in weak spirits of wine extremely useful. 
The stamen deviates in a greater degree than any other 
organ from the structure of the leaf, from a modification of 
which it is produced; and, at first sight, in many cases, it 
appears impossible to discover any analogy between the type 
and its modification ; as, for instance, between the stamen and 
leaf of a Rose. Nevertheless, if we watch the transitions that 
take place between the several organs in certain species, what 
was before mysterious or even inscrutable becomes clear and 
intelligible. In Nymphaea alba the petals so gradually change 
into stamens, that the process may be distinctly seen to depend 
