STRUCTURE.] POLLEN TUBES. 361 



in a very conspicuous manner. In consequence of their 

 manifest motion it has been conjectured that the larger par- 

 ticles 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-tube thus formed was first observed by Amici, and 

 is now known to be constantly produced at the period of 

 impregnation. Of the important offices these tubes have to 

 perform an account will be given in Book II. Chap. vi. 



For further information concerning pollen the reader is 

 referred to the following works : 



1. Fritzsche, De Plantarum Polline : Berolini, 1833. This 

 ingenious observer found that several modes of examining 

 pollen are preferable to those usually employed : 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 spon- 

 taneous 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 pollen is 

 either too much or too little transparent to show the aper- 

 tures in its sides, he finds a solution of iodine in weak spirits 

 of wine extremely useful. 2. Mohl, Sur la Structure et les 

 Formes des Grains de Pollen : translated from the German in 



