HYBRIDISING AND CROSS-BREEDING. 



of Botany,' p. 778, thus describes how fertilisation is effected : 

 " When the pollen falls upon the stigma (the tissue of which 

 at this period, as well as that forming the conducting tissue of 

 the style and neighbouring parts, secretes a peculiar viscid 

 fluid), its intine protrudes through one or more of the pores 

 or slits of the extine in the form of a delicate tube, which 

 penetrates through the cells of the stigma, by the viscid 

 secretion of which it is nourished. In most plants but one 

 pollen-tube is emitted by each pollen-cell, but the number 

 varies. . . . The pollen-tube continues to elongate by growth 

 at its apex, and passes down through the conducting tissue of 

 the canal of the style, when this exists, or directly into the 

 ovary when it is absent. This growth of the tube was formerly 

 supposed to be due to endosmotic action occurring between 

 the contents of the pollen and the secretion of the stigma and 

 style; but it is now known to be a true growth, which is 

 occasioned by the nourishing viscid secre- 

 tion which it meets with in its passage 

 through the stigma and style. 



" These tubes are extremely thin. They 

 vary in length according to circumstances, 

 but are frequently many inches ; and, as has 

 been shown by Dr Martin Duncan, they 

 are not in all cases continuous tubes, as 

 has been supposed but in Tigridia and 

 some other monocotyledonous plants they 

 are composed of several elongated cells, 

 which are, doubtless, produced by the or- 

 dinary process of cell-division. The time 

 required for the development of these tubes 

 also varies in different pollens ; thus some- 

 ^irnes they are developed almost immedi- 

 ately the pollen comes in contact with 

 the stigma, whilst in other cases many 

 hours are required for the purpose. The 

 pollen-tubes also occupy a varying time in 

 traversing the canal of the style that is, from a few hours to 

 some weeks. When the pollen-tubes have penetrated the stig- 

 matic tissue, the secretion of the latter ceases, and the stigma 

 withers. The upper part of the pollen-tube also withers above 

 as growth takes place below. 



" The pollen-tubes, having reached the ovary, are distributed 

 to the placenta or placentas, and then come in contact with 

 the ovule or ovules. One (sometimes two) of these pollen- 

 tubes enters into the micropyle of each of the ovules, and thus 



Longitudinal section 

 of a fragment of the 

 stigma of Matthiola 

 annua, Sweet (the " ten 

 weeks' stock"), showing 

 some pollen- grains, p, 

 which have emitted 

 their tube tp. t Proper 

 tissue of the stigma 

 (after Tula 

 163. 



lasne) mag. 



