EEPfiODUCTION OF ANGIOSPEEMIA. 



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

 style and neighbouring parts, secretes a peculiar viscid fluid 

 [p. 266]), 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 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 secretion which it meets with in its passage 

 through the stigma and style. 



These tubes are extremely thin. They vary in length according 



Fig. 1123. 



Fia. 1124. 



Pigr. 1123. Section of the ovule of a species of (Enoi/iera. ^ Enlarged end 

 of pollen-tube containing fovil la, which has entered the micropyle, and 

 is seen pressing inwards the apex of the embryo-sac. sr. Impregnated 

 germinal vesicle, which already begins to exhibit two parts ; one, the 

 upper, forming a suspensor, s, and another, below, r, a globular body, 

 ■which ultimately becomes the embryo, e. Endosperm cells or albumen. 



Fig. 1124. Section of the ovule of a species of Orchis, t. Enlarged 



end of the pollen-tube which has passed through the micropyle, and is 

 closely applied to the embryo-sac, the upper side of which it has pushed 

 inwards, e. Germinal vesicle in the interior of the embryo-sac in an 

 impregnated state, and dividing into two portions, the lower of which 

 is the rudimentary embryo, and the upper forms a suspensor. 



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 had been supposed, but in Tigridia and 

 some other monocotyledonous plants (see p. 257), they are 



