760 PHYSIOLOGY. 



physiologists, as to the contents of the embryo-sac previous to 

 impregnation. Schleiden, Schacht, and others, contended, that 

 no germinal vesicle existed in the sac until after the contact of 

 the pollen-tube with it in the ordinary process of impregnation; 

 in fact, they believed that, the germinal vesicle was itself formed 

 from the end of the pollen-tube, which, according to their obser- 

 vations, penetrated the wall of the sac, and by subsequent de- 

 velopment produced the embryo. If such had been proved to 

 be true, the sexuality of Phanerogamous plants would have 

 been shown to be incorrect ; but such a view was at once 

 combated by many accurate observers, who all agreed in de- 

 scribing the presence of one or more germinal vesicles or cor- 

 puscles in the sac before impregnation. Indeed, Schleiden 

 himself, who originated this view of the origin of the embryo, 

 has been convinced of his error, by Eaddlkofer, one of his own 

 distinguished pupils, so we may now consider the point as 

 settled. 



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 

 [p. 272]), its intine protrudes through one or more of the 

 pores or slits of the extine {fig. 561) 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, 

 and according to some observers, is sometimes twenty or more. 

 The pollen-tube continues to elongate by growth at its apex, 

 and passes down through the conducting tissue of the canal of 

 the style {fig. 562, tp) when this exists, or directly into the 

 ovary Avhen 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 — a kind of germination, 

 which is occasioned by the nourishing viscid secretion which it 

 meets with in its passage through the stigma and style. 



These tubes arc extremely thin. They vary in length ac- 

 cording to circumstances, but are frequently many inches. 

 The time required for their development also varies in dif- 

 ferent pollens ; thus, sometimes the pollcn-tubcs are developed 

 almost immediately the pollen comes into 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-tubes also wither above, 

 as growth takes place below. 



The pollcn-tubcs having reached the ovary arc distributed to 



