ORGANS OF REPRODUCTIOX. 



257 



the contents of the pollen-cell in the form of a jet {fg. 560). 

 These changes will take place more rapidly if a little sulphuric 

 or nitric acid be first added to the water. 



When the pollen is thrown upon the stigma under natural 

 circumstances at the period of dehiscence of the imthers, the 

 above-described action becomes materially modified. In this case 

 the pollen-cell does not burst, but its intine protrudes through 

 one or more of the piores or slits of the extine in the form 

 of a delicate tube {fys. 561 and 562, tp), filled with the fovilla, 



Fig. 560. 



Fig. 561. 



Fig. 562. 



F(g.560. Pollen oFCherrj- dischargincr it? fovilla througli an openingr in 



the intine. Fig. 561. Trigonal pollen of CEiiathera with a pollen-rnbe. 



Fig. 562. Vertical sertiou of the stiarnia and part of the style nf An- 

 tirrhinum rnajus. stig. StiKHia, on which two pollencplls have fallen, 

 each of which is provided %vith a poUeu-tube, tp, which pierces the 

 tissue of the style, styl, 



and called the pollen-tube ; this penetrates (as will be afterwards 

 described) through the tissue of the stigma, and style, also, when 

 this is present, to the placenta and ovules. This tube is fre- 

 quently some inches in length, and its formation is not due, as 

 was formerly supposed, to endosmotic action, but it is a true 

 growth like that of the radicle of the embryo in the process of 

 germination, and caused by the noiu-ishment it derives from the 

 stigma and conducting tissue of the style. (See page 23o.) 



Dr. P. Martin Duncan has recently proved that the pollen- 

 tube is not (in all cases at least), as was before common!}^ sup- 

 posed, a continuous tube, that is, ha^^ng but one cavity ; but 

 that in J'igridia conchifiora and all other monocotyledons which 

 he has examined with long styles, " transverse inflections of the 

 tubular cell-wall of the pollen-tube exist every now and then ; " 

 so that then " the pollen-tube is really a tube formed by elon- 

 gated cells." Dr. Duncan also informs me that, so far as he has 

 examined, the pollen-tube in all Dicotyledons is continuous. 



