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 jjlantarum polline 

 Berolini, 1833. This ingenious observer found that several 

 modes of examining pollen are preferable to those usually em- 

 ployed. In particular he recominends 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 Nymphsea alba the petals so gradually change 

 into stamens, that the process may be distinctly seen to depend 



